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/locales/en.json

@@ -4810,6 +4810,13 @@
       "openBot": "Open the bot",
       "retry": "I pressed /start, retry",
       "hint": "After running /start, return here and tap «Retry»."
+    },
+    "serviceUnavailable": {
+      "title": "Service unavailable",
+      "description": "We can't reach the service right now. It may be undergoing maintenance or there's a temporary connection issue.",
+      "retry": "Try again",
+      "checking": "Checking connection...",
+      "hint": "This page will refresh automatically as soon as the service is back."
     }
   },
   "merge": {