fix(pwa): stay in server mode when a known server does not answer

A failed start-up probe used to fall back to browser mode, which showed an
empty library while the backend restarted and let new playlists land in the
browser. The browser now remembers the server; without an answer it starts
in server mode, offline, and reloads once the server is back. Settings →
Server shows the backend address instead of the page's.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture/pwa-self-hosted.md

@@ -170,7 +170,13 @@ expose desktop-only actions in the PWA/shared UI.
 deployment wrote a runtime `BACKEND_URL` into `window.__IPTVNATOR_CONFIG__`
 (the Docker entrypoint does; hosted builds and Electron never probe). On
 `storage: 'server'` it imports `apps/web/src/app/server-storage/` lazily and
-merges `provideServerStorage()` into the app config. Everything else of the
+merges `provideServerStorage()` into the app config, and remembers the
+instance under `iptvnator-server:instance`. When the probe gets no answer
+(network error, timeout, or a 5xx such as nginx's 502 while the backend
+restarts), a browser that remembers this backend still starts in server
+mode, offline, and reloads once the server answers; it never falls back to
+an empty browser library. An answer without server storage, or a 404 from
+an older backend, forgets it (`server-storage-start.ts`). Everything else of the
 client lives behind the `@iptvnator/services/server-storage` sub-entry, so
 other builds never load it; `ServerStorageStatusService` and the token
 helpers stay in the main `@iptvnator/services` barrel because the rail,