feat(desktop): show the AI assistant in SaaS mode via the cloud kill switch (#6666)

## What & why

Chained on top of #6649 (the `cloud/` refactor). The AI assistant was
effectively dead on desktop:

1. **Hidden** — `ChatFAB` gates on `aiEngineEnabled`, which desktop
reads from the **local** bundled backend's `/api/v1/config/app-config`.
The local backend has no AI engine, so the flag is always `false` and
the FAB never renders.
2. **Mis-routed** — even if shown, AI calls used `getApiBaseUrl()`,
which is empty/local on desktop, so the orchestrate stream and AI
result-file download missed the engine (which only runs in the cloud).

This PR wires AI properly **without hardcoding it on**, so the cloud
keeps the kill switch: flip `aiEngineEnabled` server-side and the
desktop FAB disappears on the next load — no desktop release required.
(Deliberately *not* assume-on, so a future "turn AI off" doesn't strand
shipped versions.)

## Changes

**General SaaS app-config service** (reusable for any cloud flag, not
just AI):
- `desktop/services/saasAppConfigService.ts` — SaaS-mode-only fetch +
5-min cache of the **public** `/api/v1/config/app-config` from the
**SaaS** backend over native HTTP (`@tauri-apps/plugin-http`, no CORS).
Returns `null` outside SaaS mode.
- `desktop/hooks/useSaasAppConfig.ts` — hook over it; reloads on
connection-mode change.

**AI gating + routing seams:**
- `useAiEngineEnabled()` — core reads `useAppConfig()` (web), desktop
reads `useSaasAppConfig()`. `ChatFAB` consumes it.
- `getAiBaseUrl()` — core uses the normal API base (web), desktop points
AI calls at the SaaS backend. `ChatContext` uses it for the orchestrate
stream + result-file download.
- `operationRouter` — route `/api/v1/ai/*` to the SaaS backend
(cloud-only prefix).

**Docs:** AGENTS.md gains a short "cloud feature flags on desktop" note
so the pattern is maintained.

## Verification
- `tsc --noEmit` green for saas / desktop / cloud flavors
- `eslint --max-warnings=0` clean (cloud-layer guardrail respected — the
platform-coupled bits live in `desktop/`)
- New `saasAppConfigService.test.ts` (3 tests) + existing
`operationRouter` / `tauriHttpClient` / `httpErrorHandler` suites green
- 0 stray compiled artifacts

## Not headlessly verifiable — needs a live Tauri smoke
The orchestrate **SSE stream** uses the webview's global `fetch` (native
HTTP can't stream the body the same way), so it's subject to browser
CORS to the SaaS backend. The `SupabaseSecurityConfig` tauri-origin
allowance (from #6649) covers it, but please confirm on a real build:
open the FAB in SaaS mode, run an agent task, watch the stream + a
result-file download succeed.

frontend/editor/src/core/hooks/useAiEngineEnabled.ts

@@ -0,0 +1,13 @@
+import { useAppConfig } from "@app/contexts/AppConfigContext";
+
+/**
+ * Whether the AI engine is enabled, per the backend's app-config.
+ *
+ * Web builds read it straight from the app-config (which already comes from the
+ * backend the app talks to). Desktop shadows this (desktop/hooks/useAiEngineEnabled)
+ * to read it from the SaaS backend in SaaS mode instead of the local bundled one.
+ */
+export function useAiEngineEnabled(): boolean {
+  const { config } = useAppConfig();
+  return Boolean(config?.aiEngineEnabled);
+}

frontend/editor/src/core/services/aiBaseUrl.ts

@@ -0,0 +1,12 @@
+import { getApiBaseUrl } from "@app/services/apiClientConfig";
+
+/**
+ * Base URL for AI-engine calls (orchestrate stream, AI result-file download).
+ *
+ * Web builds talk to whichever backend the app is served from, so the normal API
+ * base is correct. Desktop shadows this (desktop/services/aiBaseUrl) to point at
+ * the SaaS backend, since the AI engine only runs in the cloud.
+ */
+export function getAiBaseUrl(): string {
+  return getApiBaseUrl();
+}

frontend/editor/src/desktop/hooks/useAiEngineEnabled.ts

@@ -0,0 +1,13 @@
+import { useSaasAppConfig } from "@app/hooks/useSaasAppConfig";
+
+/**
+ * Desktop: the AI engine runs on the SaaS backend, so its enabled flag must come
+ * from the SaaS app-config (not the local bundled backend, which never has the
+ * engine). useSaasAppConfig() returns null outside SaaS mode, so AI is implicitly
+ * hidden in local/self-hosted — and the cloud retains the on/off switch (flip
+ * aiEngineEnabled server-side and the desktop FAB disappears on next load, no
+ * release required).
+ */
+export function useAiEngineEnabled(): boolean {
+  return Boolean(useSaasAppConfig()?.aiEngineEnabled);
+}

frontend/editor/src/desktop/hooks/useSaasAppConfig.ts

@@ -0,0 +1,36 @@
+import { useEffect, useState } from "react";
+import { saasAppConfigService } from "@app/services/saasAppConfigService";
+import { connectionModeService } from "@app/services/connectionModeService";
+import type { AppConfig } from "@app/types/appConfig";
+
+/**
+ * The SaaS backend's app-config while in desktop SaaS mode, or null otherwise.
+ *
+ * General-purpose: read any cloud feature flag from it (e.g.
+ * `useSaasAppConfig()?.aiEngineEnabled`, `?.premiumEnabled`). Reloads when the
+ * connection mode changes, so switching into/out of SaaS updates the flags
+ * (and a server-side flag flip is picked up on the next load).
+ */
+export function useSaasAppConfig(): AppConfig | null {
+  const [config, setConfig] = useState<AppConfig | null>(null);
+
+  useEffect(() => {
+    let alive = true;
+    const load = () => {
+      void saasAppConfigService.getConfig().then((c) => {
+        if (alive) setConfig(c);
+      });
+    };
+    load();
+    const unsubscribe = connectionModeService.subscribeToModeChanges(() => {
+      saasAppConfigService.clearCache();
+      load();
+    });
+    return () => {
+      alive = false;
+      unsubscribe?.();
+    };
+  }, []);
+
+  return config;
+}

frontend/editor/src/desktop/services/aiBaseUrl.ts

@@ -0,0 +1,14 @@
+import { STIRLING_SAAS_BACKEND_API_URL } from "@app/constants/connection";
+
+/**
+ * Desktop: AI-engine calls must hit the SaaS backend (the local bundled backend
+ * has no AI engine). The AI surface is only shown in SaaS mode (gated by
+ * useAiEngineEnabled), so returning the SaaS base unconditionally is correct —
+ * when an AI call is made, the app is in SaaS mode.
+ *
+ * Used for the orchestrate stream (a raw fetch) and the AI result-file download,
+ * which would otherwise resolve to the empty/local base and miss the engine.
+ */
+export function getAiBaseUrl(): string {
+  return (STIRLING_SAAS_BACKEND_API_URL ?? "").replace(/\/$/, "");
+}

frontend/editor/src/desktop/services/apiClientSetup.test.ts

@@ -6,6 +6,11 @@ import { expectConsole } from "@app/tests/failOnConsole";
 // Exercise the error-interceptor logic against a hand-rolled axios-like
 // client; transitive imports are mocked out so this is a pure unit test.
 
+const { getAccessTokenMock } = vi.hoisted(() => ({
+  getAccessTokenMock: vi.fn(),
+}));
+
+vi.mock("@app/auth/session", () => ({ getAccessToken: getAccessTokenMock }));
 vi.mock("@app/components/toast", () => ({ alert: vi.fn() }));
 vi.mock("@core/services/apiClientSetup", () => ({
   setupApiInterceptors: vi.fn(),
@@ -50,7 +55,11 @@ vi.mock("@app/i18n", () => ({
   default: { t: (_key: string, fallback: string) => fallback || _key },
 }));
 
-import { setupApiInterceptors } from "@app/services/apiClientSetup";
+import {
+  setupApiInterceptors,
+  getAuthHeaders,
+} from "@app/services/apiClientSetup";
+import { operationRouter } from "@app/services/operationRouter";
 
 type Interceptor = (value: unknown) => unknown;
 
@@ -177,3 +186,66 @@ describe("desktop apiClientSetup - 401 silent-path", () => {
     expect(events).toHaveLength(0);
   });
 });
+
+describe("desktop request interceptor - auth for SaaS-backend requests", () => {
+  type ReqConfig = {
+    url: string;
+    method: string;
+    headers: Record<string, string>;
+  };
+
+  async function runRequestInterceptor(config: ReqConfig): Promise<ReqConfig> {
+    const { client, handlers } = makeMockClient();
+    setupApiInterceptors(client as unknown as AxiosInstance);
+    const handler = handlers.request[0];
+    expect(handler).toBeTypeOf("function");
+    return (await handler(config)) as ReqConfig;
+  }
+
+  beforeEach(() => {
+    getAccessTokenMock.mockReset();
+    // SaaS mode (not self-hosted), local-first routing for tool paths.
+    vi.mocked(operationRouter.isSelfHostedMode).mockResolvedValue(false);
+    vi.mocked(operationRouter.isSaaSMode).mockResolvedValue(true);
+    vi.mocked(operationRouter.getBaseUrl).mockResolvedValue(
+      "http://localhost:8080",
+    );
+    vi.mocked(operationRouter.shouldSkipBackendReadyCheck).mockResolvedValue(
+      true,
+    );
+  });
+
+  test("attaches the Bearer token to an absolute SaaS-backend URL (AI file download)", async () => {
+    getAccessTokenMock.mockResolvedValue("jwt-123");
+    const result = await runRequestInterceptor({
+      url: "https://api.saas.test/api/v1/general/files/abc",
+      method: "get",
+      headers: {},
+    });
+    expect(result.headers.Authorization).toBe("Bearer jwt-123");
+  });
+
+  test("does NOT attach a token to a relative, local-routed tool request", async () => {
+    getAccessTokenMock.mockResolvedValue("jwt-123");
+    const result = await runRequestInterceptor({
+      url: "/api/v1/general/merge-pdfs",
+      method: "get",
+      headers: {},
+    });
+    expect(result.headers.Authorization).toBeUndefined();
+  });
+});
+
+describe("desktop getAuthHeaders (raw fetch / AI SSE stream)", () => {
+  beforeEach(() => getAccessTokenMock.mockReset());
+
+  test("attaches the Tauri-store JWT as a Bearer header", async () => {
+    getAccessTokenMock.mockResolvedValue("jwt-123");
+    expect(await getAuthHeaders()).toEqual({ Authorization: "Bearer jwt-123" });
+  });
+
+  test("returns no header when there is no token", async () => {
+    getAccessTokenMock.mockResolvedValue(null);
+    expect(await getAuthHeaders()).toEqual({});
+  });
+});

frontend/editor/src/desktop/services/apiClientSetup.ts

@@ -1,11 +1,6 @@
 import type { AxiosInstance, InternalAxiosRequestConfig } from "axios";
 import { alert } from "@app/components/toast";
-import {
-  setupApiInterceptors as coreSetup,
-  getAuthHeaders,
-} from "@core/services/apiClientSetup";
-
-export { getAuthHeaders };
+import { setupApiInterceptors as coreSetup } from "@core/services/apiClientSetup";
 import { tauriBackendService } from "@app/services/tauriBackendService";
 import { createBackendNotReadyError } from "@app/constants/backendErrors";
 import { operationRouter } from "@app/services/operationRouter";
@@ -19,6 +14,21 @@ import {
 import { OPEN_SIGN_IN_EVENT } from "@app/constants/signInEvents";
 import i18n from "@app/i18n";
 
+/**
+ * Auth headers for raw fetch() calls (the AI SSE stream) — desktop variant.
+ *
+ * Core's getAuthHeaders returns {} and proprietary's reads the JWT from
+ * localStorage, but desktop keeps its JWT in the Tauri secure store. So this
+ * pulls the token via getAccessToken() (mirroring the axios request interceptor,
+ * including waiting out an in-flight refresh) — without it the orchestrate
+ * stream hits the SaaS backend with no Authorization header and 401s.
+ */
+export async function getAuthHeaders(): Promise<Record<string, string>> {
+  await authService.awaitRefreshIfInProgress();
+  const token = await getAccessToken();
+  return token ? { Authorization: `Bearer ${token}` } : {};
+}
+
 const BACKEND_TOAST_COOLDOWN_MS = 4000;
 let lastBackendToast = 0;
 
@@ -71,7 +81,17 @@ export function setupApiInterceptors(client: AxiosInstance): void {
         // - SaaS backend: Needs auth token
         // - Self-hosted backend: Needs auth token
         const isRemote = await operationRouter.isSelfHostedMode();
-        const isSaaSBackendRequest = baseUrl === STIRLING_SAAS_BACKEND_API_URL;
+        // A request targets the SaaS backend either because operationRouter
+        // routed a relative path there, OR because the caller passed an absolute
+        // SaaS URL. The latter matters for the AI result-file download: it hits
+        // ${getAiBaseUrl()}/api/v1/general/files/..., and /api/v1/general/*
+        // normally routes local-first — so without this it would reach the cloud
+        // engine with no token and 401. Check the FINAL url (post-prefixing).
+        const saasBase = STIRLING_SAAS_BACKEND_API_URL ?? "";
+        const finalUrl = extendedConfig.url ?? "";
+        const isSaaSBackendRequest =
+          baseUrl === STIRLING_SAAS_BACKEND_API_URL ||
+          (saasBase !== "" && finalUrl.startsWith(saasBase));
         const needsAuth = isRemote || isSaaSBackendRequest;
 
         // Tag request so error handler can identify SaaS backend errors without URL matching

frontend/editor/src/desktop/services/operationRouter.ts

@@ -60,6 +60,7 @@ export class OperationRouter {
       /^\/api\/v1\/auth\//, // Supabase auth (SaaS mode)
       /^\/api\/v1\/payg\//, // PAYG wallet / spend-cap / billing
       /^\/api\/v1\/policies(?:\/|$)/, // Policy runs — must bill via the cloud
+      /^\/api\/v1\/ai\//, // AI engine (orchestrate, etc.) — runs in the cloud
       // Add more cloud-only feature prefixes here as they land.
     ];
 

frontend/editor/src/desktop/services/saasAppConfigService.test.ts

@@ -0,0 +1,64 @@
+import { describe, expect, test, vi, beforeEach } from "vitest";
+import { expectConsole } from "@app/tests/failOnConsole";
+
+const { fetchMock, getModeMock } = vi.hoisted(() => ({
+  fetchMock: vi.fn(),
+  getModeMock: vi.fn(),
+}));
+vi.mock("@tauri-apps/plugin-http", () => ({ fetch: fetchMock }));
+vi.mock("@app/constants/connection", () => ({
+  STIRLING_SAAS_BACKEND_API_URL: "https://api.saas.test",
+}));
+vi.mock("@app/services/connectionModeService", () => ({
+  connectionModeService: { getCurrentMode: getModeMock },
+}));
+
+import { saasAppConfigService } from "@app/services/saasAppConfigService";
+
+function okConfig(body: Record<string, unknown>) {
+  return { ok: true, status: 200, json: async () => body };
+}
+
+describe("saasAppConfigService", () => {
+  beforeEach(() => {
+    fetchMock.mockReset();
+    getModeMock.mockReset();
+    saasAppConfigService.clearCache();
+  });
+
+  test("returns null outside SaaS mode without fetching", async () => {
+    getModeMock.mockResolvedValue("local");
+    expect(await saasAppConfigService.getConfig()).toBeNull();
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+
+  test("fetches the SaaS app-config in SaaS mode and caches it", async () => {
+    getModeMock.mockResolvedValue("saas");
+    fetchMock.mockResolvedValue(
+      okConfig({ aiEngineEnabled: true, premiumEnabled: false }),
+    );
+
+    const first = await saasAppConfigService.getConfig();
+    expect(first?.aiEngineEnabled).toBe(true);
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+    expect(fetchMock.mock.calls[0][0]).toBe(
+      "https://api.saas.test/api/v1/config/app-config",
+    );
+
+    // Second read is served from cache (no extra fetch).
+    const second = await saasAppConfigService.getConfig();
+    expect(second?.aiEngineEnabled).toBe(true);
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+  });
+
+  test("returns null on a non-ok response", async () => {
+    getModeMock.mockResolvedValue("saas");
+    fetchMock.mockResolvedValue({
+      ok: false,
+      status: 500,
+      json: async () => ({}),
+    });
+    expectConsole.warn(/SaaS app-config fetch failed: 500/);
+    expect(await saasAppConfigService.getConfig()).toBeNull();
+  });
+});

frontend/editor/src/desktop/services/saasAppConfigService.ts

@@ -0,0 +1,82 @@
+import { fetch } from "@tauri-apps/plugin-http";
+import { STIRLING_SAAS_BACKEND_API_URL } from "@app/constants/connection";
+import { connectionModeService } from "@app/services/connectionModeService";
+import type { AppConfig } from "@app/types/appConfig";
+
+/**
+ * General fetch+cache of the SaaS backend's app-config for the desktop app.
+ *
+ * The local AppConfigContext reads /api/v1/config/app-config from the LOCAL
+ * bundled backend (that's what apiClient routes to on desktop), so cloud-only
+ * feature flags (aiEngineEnabled, premiumEnabled, paygEnabled, ...) are never
+ * seen. This service fetches the SAME endpoint from the SaaS backend so desktop
+ * SaaS mode can read the cloud's view of those flags. It is intentionally
+ * generic — not AI-specific — so other desktop SaaS features can consume it.
+ *
+ * - Only returns config in SaaS connection mode; null in local/self-hosted (so
+ *   callers naturally treat cloud features as off outside SaaS, and a server-side
+ *   flag flip — e.g. turning AI off — propagates without a desktop release).
+ * - The endpoint is public on the SaaS backend (/api/v1/config/** is permitAll),
+ *   so no auth is required.
+ * - Native HTTP (@tauri-apps/plugin-http) — no CORS, mirrors endpointAvailabilityService.
+ */
+const CACHE_MS = 5 * 60 * 1000;
+
+class SaasAppConfigService {
+  private cache: AppConfig | null = null;
+  private cacheExpiry = 0;
+  private inFlight: Promise<AppConfig | null> | null = null;
+
+  /** The SaaS backend's app-config, or null outside SaaS mode / on failure. */
+  async getConfig(force = false): Promise<AppConfig | null> {
+    const mode = await connectionModeService.getCurrentMode();
+    if (mode !== "saas" || !STIRLING_SAAS_BACKEND_API_URL) {
+      this.clearCache();
+      return null;
+    }
+    if (!force && this.cache && Date.now() < this.cacheExpiry) {
+      return this.cache;
+    }
+    if (this.inFlight) return this.inFlight;
+    this.inFlight = this.fetchConfig();
+    try {
+      return await this.inFlight;
+    } finally {
+      this.inFlight = null;
+    }
+  }
+
+  private async fetchConfig(): Promise<AppConfig | null> {
+    try {
+      const base = STIRLING_SAAS_BACKEND_API_URL.replace(/\/$/, "");
+      const response = await fetch(`${base}/api/v1/config/app-config`, {
+        method: "GET",
+        headers: { "Cache-Control": "no-store" },
+      });
+      if (!response.ok) {
+        console.warn(
+          `[saasAppConfigService] SaaS app-config fetch failed: ${response.status}`,
+        );
+        return null;
+      }
+      const data = (await response.json()) as AppConfig;
+      this.cache = data;
+      this.cacheExpiry = Date.now() + CACHE_MS;
+      return data;
+    } catch (error) {
+      console.error(
+        "[saasAppConfigService] SaaS app-config fetch error:",
+        error,
+      );
+      return null;
+    }
+  }
+
+  /** Drop the cache (e.g. on connection-mode change) so the next read re-fetches. */
+  clearCache(): void {
+    this.cache = null;
+    this.cacheExpiry = 0;
+  }
+}
+
+export const saasAppConfigService = new SaasAppConfigService();

frontend/editor/src/proprietary/components/chat/ChatContext.tsx

@@ -10,7 +10,7 @@ import { useTranslation } from "react-i18next";
 import { generateId } from "@app/utils/generateId";
 import { useAllFiles, useFileActions } from "@app/contexts/FileContext";
 import apiClient from "@app/services/apiClient";
-import { getApiBaseUrl } from "@app/services/apiClientConfig";
+import { getAiBaseUrl } from "@app/services/aiBaseUrl";
 import { getAuthHeaders } from "@app/services/apiClientSetup";
 import { dispatchPaygLimitReached } from "@app/services/usageLimitBridge";
 import { createChildStub } from "@app/contexts/file/fileActions";
@@ -407,8 +407,10 @@ export function ChatProvider({ children }: { children: ReactNode }) {
   // Download a File from the Stirling files endpoint.
   const downloadFile = useCallback(
     async (descriptor: AiWorkflowResultFile): Promise<File> => {
+      // AI result files live on the backend that ran the workflow (the SaaS
+      // engine on desktop), so fetch from the AI base, not the local backend.
       const response = await apiClient.get<Blob>(
-        `/api/v1/general/files/${descriptor.fileId}`,
+        `${getAiBaseUrl()}/api/v1/general/files/${descriptor.fileId}`,
         { responseType: "blob" },
       );
       return new File([response.data], descriptor.fileName, {
@@ -520,7 +522,7 @@ export function ChatProvider({ children }: { children: ReactNode }) {
           formData.append(`conversationHistory[${i}].content`, message.content);
         });
         const response = await fetch(
-          `${getApiBaseUrl()}/api/v1/ai/orchestrate/stream`,
+          `${getAiBaseUrl()}/api/v1/ai/orchestrate/stream`,
           {
             method: "POST",
             body: formData,

frontend/editor/src/proprietary/components/chat/ChatFAB.tsx

@@ -6,7 +6,7 @@ import { ChatFABButton } from "@shared/components/ChatFABButton";
 import { ChatFABWindow } from "@shared/components/ChatFABWindow";
 import { ChatPanel } from "@app/components/chat/ChatPanel";
 import { useChat } from "@app/components/chat/ChatContext";
-import { useAppConfig } from "@app/contexts/AppConfigContext";
+import { useAiEngineEnabled } from "@app/hooks/useAiEngineEnabled";
 import { Z_INDEX_CHAT_FAB_OVERLAY } from "@app/styles/zIndex";
 import "@app/components/chat/ChatFAB.css";
 
@@ -53,8 +53,9 @@ export function ChatFAB() {
   const [isOpen, setIsOpen] = useState(false);
   const [hasUnviewedResult, setHasUnviewedResult] = useState(false);
   const { isLoading } = useChat();
-  const { config } = useAppConfig();
-  const enabled = Boolean(config?.aiEngineEnabled);
+  // Desktop sources this from the SaaS backend (cloud kill switch); web reads it
+  // from the local app-config. Either way the AI engine drives FAB visibility.
+  const enabled = useAiEngineEnabled();
 
   // Detect loading → done transition. If the FAB is closed when the agent
   // finishes, show the tick badge until the user opens the panel.