Desktop: Fallback to local backend if self-hosted server is offline (#5880)

* Adds a fallback mechanism so the desktop app routes tool operations to
the local bundled backend when the user's self-hosted Stirling-PDF
server goes offline, and disables tools in the UI that aren't supported
locally.

* `selfHostedServerMonitor.ts` independently polls the self-hosted
server every 15s and exposes which tool endpoints are unavailable when
it goes offline
* `operationRouter.ts` intercepts operations destined for the
self-hosted server and reroutes them to the local bundled backend when
the monitor reports it offline
* `useSelfHostedToolAvailability.ts` feeds the offline tool set into
useToolManagement, disabling affected tools in the UI with a
selfHostedOffline reason and banner warning

- `SelfHostedOfflineBanner `is a dismissable (session-only) gray bar
shown at the top of the UI when in self-hosted mode and the server goes
offline. It shows:

frontend/src/desktop/hooks/useBackendHealth.ts

@@ -1,24 +1,63 @@
 import { useEffect, useState, useCallback } from 'react';
 import { backendHealthMonitor } from '@app/services/backendHealthMonitor';
+import { selfHostedServerMonitor } from '@app/services/selfHostedServerMonitor';
+import { tauriBackendService } from '@app/services/tauriBackendService';
+import { connectionModeService } from '@app/services/connectionModeService';
 import type { BackendHealthState } from '@app/types/backendHealth';
 
 /**
- * Hook to read the shared backend health monitor state.
- * All consumers subscribe to a single poller managed by backendHealthMonitor.
+ * Hook to read backend health state for UI (Run button, BackendHealthIndicator).
+ *
+ * backendHealthMonitor tracks the local bundled backend.
+ * selfHostedServerMonitor tracks the remote server in self-hosted mode.
+ *
+ * isOnline logic:
+ * - SaaS mode: true when local backend is healthy
+ * - Self-hosted mode (server online): true (remote is up)
+ * - Self-hosted mode (server offline, local port known): true so the Run button
+ *   stays enabled — operationRouter routes supported tools to local
+ * - Self-hosted mode (server offline, local port unknown): false
  */
 export function useBackendHealth() {
   const [health, setHealth] = useState<BackendHealthState>(() => backendHealthMonitor.getSnapshot());
+  const [serverStatus, setServerStatus] = useState(
+    () => selfHostedServerMonitor.getSnapshot().status
+  );
+  const [localUrl, setLocalUrl] = useState<string | null>(
+    () => tauriBackendService.getBackendUrl()
+  );
+  const [connectionMode, setConnectionMode] = useState<string | null>(null);
+
+  useEffect(() => {
+    void connectionModeService.getCurrentMode().then(setConnectionMode);
+  }, []);
 
   useEffect(() => {
     return backendHealthMonitor.subscribe(setHealth);
   }, []);
 
+  useEffect(() => {
+    return selfHostedServerMonitor.subscribe(state => setServerStatus(state.status));
+  }, []);
+
+  useEffect(() => {
+    return tauriBackendService.subscribeToStatus(() => {
+      setLocalUrl(tauriBackendService.getBackendUrl());
+    });
+  }, []);
+
   const checkHealth = useCallback(async () => {
     return backendHealthMonitor.checkNow();
   }, []);
 
+  const isOnline =
+    connectionMode === 'selfhosted'
+      ? serverStatus !== 'offline' || !!localUrl
+      : health.isOnline;
+
   return {
     ...health,
+    isOnline,
     checkHealth,
   };
 }