fix(server-storage): keep Xtream data, ordered sync, sign-in and stale-cache fixes

- Moving browser playlists matches them to the server's by id or source and
  carries the legacy Xtream maps under the server id; it no longer goes
  through the backup format, whose PWA export has no Xtream user data. A
  partial move stays on offer.
- User-data batches go out in order, and a re-read keeps entries edited
  while it was in flight.
- A keepPayload write keeps its cached copy only when the server replaced
  the version its payload came from (new X-Replaced-ETag header).
- 502/503/504 from the proxy count as offline.
- The start-up screen lets the sign-in page through while a token is
  required.
- Settings → Server offers a reload after a move.
- The probe detects Electron through RuntimeCapabilitiesService.

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

apps/web/src/app/settings/settings-server-section.component.spec.ts

@@ -92,6 +92,7 @@ describe('SettingsServerSectionComponent', () => {
 
         expect(migration.moveBrowserData).toHaveBeenCalled();
         expect(byTestId('server-import-browser')).toBeNull();
+        expect(byTestId('server-import-reload')).not.toBeNull();
         expect(fixture.nativeElement.textContent).toContain(
             'SERVER_STORAGE.MIGRATION_DONE'
         );