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.

AGENTS.md

@@ -193,6 +193,8 @@ What goes where:
 
 Rule of thumb — **move, don't copy**: share via `cloud/`, override by shadowing the same `@app/*` path in a leaf (`saas/` or `desktop/`).
 
+**Cloud feature flags on desktop.** The local `AppConfigContext` reads `/api/v1/config/app-config` from the LOCAL bundled backend, so cloud-only flags (`aiEngineEnabled`, `premiumEnabled`, …) are never seen on desktop. To read the cloud's view, use `useSaasAppConfig()` (`desktop/hooks/useSaasAppConfig.ts`, backed by the general `saasAppConfigService` — SaaS-mode-only, public endpoint, native HTTP, 5-min cache). It returns `null` outside SaaS mode, so cloud features stay off in local/self-hosted and the server keeps the on/off switch (no desktop release needed to flip a flag). Gate a feature behind a per-platform seam — e.g. `useAiEngineEnabled()` (core reads `useAppConfig()`, desktop reads `useSaasAppConfig()`) — rather than hardcoding the flag on.
+
 #### Component Override Pattern (Stub/Shadow)
 Use this pattern for desktop-specific or proprietary-specific features WITHOUT runtime checks or conditionals.