Add SaaS frontend code (#5879)

# Description of Changes
Adds the code for the SaaS frontend as proprietary code to the OSS repo.
This version of the code is adapted from 22/1/2026, which was the last
SaaS version based on the 'V2' design. This will move us closer to being
able to have the OSS products understand whether the user has a SaaS
account, and provide the correct UI in those cases.

frontend/src/core/hooks/tools/shared/useToolOperation.ts

@@ -121,13 +121,11 @@ export const useToolOperation = <TParams>(
         ? config.endpoint(params)
         : config.endpoint;
 
-    // Credit check for cloud operations (desktop SaaS mode only, no-op in web builds)
-    if (willUseCloud && endpoint) {
-      const creditError = await checkCredits();
-      if (creditError !== null) {
-        actions.setError(creditError);
-        return;
-      }
+    // Credit check — no-op in core builds, real check in desktop/SaaS versions
+    const creditError = await checkCredits();
+    if (creditError !== null) {
+      actions.setError(creditError);
+      return;
     }
 
     // Backend readiness check (will skip for SaaS-routed endpoints)

frontend/src/core/hooks/useConfigButtonIcon.tsx

@@ -0,0 +1,6 @@
+/**
+ * Core stub — config button uses the default settings icon.
+ */
+export function useConfigButtonIcon(): React.ReactNode {
+  return null;
+}

frontend/src/core/hooks/useJwtConfigSync.ts

@@ -0,0 +1,26 @@
+import { useEffect } from 'react';
+
+/**
+ * Core implementation: sets up the jwt-available event listener for OSS JWT auth,
+ * and detects auth pages where config fetch should be skipped.
+ */
+export function useJwtConfigSync(fetchConfig: (force?: boolean) => void): { isAuthPage: boolean } {
+  const currentPath = window.location.pathname;
+  const isAuthPage =
+    currentPath.includes('/login') ||
+    currentPath.includes('/signup') ||
+    currentPath.includes('/auth/callback') ||
+    currentPath.includes('/invite/');
+
+  useEffect(() => {
+    const handleJwtAvailable = () => {
+      console.debug('[AppConfig] JWT available event - refetching config');
+      fetchConfig(true);
+    };
+
+    window.addEventListener('jwt-available', handleJwtAvailable);
+    return () => window.removeEventListener('jwt-available', handleJwtAvailable);
+  }, [fetchConfig]);
+
+  return { isAuthPage };
+}