feat(cabinet): recoverable "service unavailable" screen when the backend is unreachable

When the backend was down the cabinet got stuck on a blank loader: the bootstrap
token refresh used bare axios with no timeout (hung forever), and every
interceptor guard required an HTTP response — so a transport-level failure had
zero handling and no error UI ever appeared.

Add a full-screen ServiceUnavailableScreen (extends the existing blocking-store
pattern) shown whenever the backend is unreachable, that auto-recovers when it
returns:
- New 'backend_unavailable' blocking type + BlockingOverlay branch; mirrors
  MaintenanceScreen, CloudWarningIcon, i18n in ru/en/zh/fa.
- api/health.ts: pingBackend() probes the root /health/unified (bypasses the
  /api baseURL + interceptor); URL derived from the origin for remote/sub-path
  deploys; 502/503/504 count as down. reportPossibleBackendDown() confirms an
  outage with a liveness probe before flipping, so a one-off blip never blanks a
  loaded app.
- Detected from the response interceptor (no-response) and the bootstrap refresh
  path; doRefresh gets a timeout so it can no longer hang.
- A transport failure during refresh now PRESERVES the session (distinguished
  from a rejected token via lastFailureWasTransport) instead of logging the user
  out, so recovery actually resumes.
- Recovery lifts the overlay + refetches for an already-loaded session, and only
  hard-reloads when the initial bootstrap never reached the backend — no lost
  form state. Manual retry + 5s auto-poll. Dev proxy for /health.

src/App.tsx

@@ -28,6 +28,7 @@ import {
   ChannelSubscriptionScreen,
   BlacklistedScreen,
   AccountDeletedScreen,
+  ServiceUnavailableScreen,
 } from './components/blocking';
 import { ErrorBoundary } from './components/ErrorBoundary';
 import { PermissionRoute } from '@/components/auth/PermissionRoute';
@@ -232,6 +233,10 @@ function BlockingOverlay() {
     return <AccountDeletedScreen />;
   }
 
+  if (blockingType === 'backend_unavailable') {
+    return <ServiceUnavailableScreen />;
+  }
+
   return null;
 }