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/components/DesktopBannerInitializer.tsx

@@ -3,6 +3,7 @@ import { useBanner } from '@app/contexts/BannerContext';
 import { DefaultAppBanner } from '@app/components/shared/DefaultAppBanner';
 import UpgradeBanner from '@app/components/shared/UpgradeBanner';
 import { TeamInvitationBanner } from '@app/components/shared/TeamInvitationBanner';
+import { SelfHostedOfflineBanner } from '@app/components/shared/SelfHostedOfflineBanner';
 
 export function DesktopBannerInitializer() {
   const { setBanner } = useBanner();
@@ -10,6 +11,7 @@ export function DesktopBannerInitializer() {
   useEffect(() => {
     setBanner(
       <>
+        <SelfHostedOfflineBanner />
         <TeamInvitationBanner />
         <UpgradeBanner />
         <DefaultAppBanner />