fix(player): request POST_NOTIFICATIONS; auto-minimize player on teardown

Device-surfaced on physical Android 13+ (worked on emulator):

A) The media notification never appeared because the app never
   requested POST_NOTIFICATIONS at runtime — the manifest declares it
   and the foreground service is correct, but Android 13+ denies it by
   default until asked. Add permission_handler ^12.0.1 and request
   Permission.notification once at startup (post-first-frame,
   Platform.isAndroid-guarded; no-op on <13 / once decided).

B) When the #52 idle/dismiss teardown nulled mediaItem while the full
   NowPlayingScreen was open, it stranded the user on an empty
   "Nothing playing." Scaffold. Now post-frame maybePop() so it
   auto-minimizes (the mini bar is already gone).

pubspec.lock + db.g.dart regenerated by CI/build.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/pubspec.yaml

@@ -25,6 +25,9 @@ dependencies:
   # Lucide icon set (design system mandates Lucide, not Material).
   # Icons exposed as LucideIcons.<snake_case> IconData usable in Icon().
   flutter_lucide: ^1.11.0
+  # Runtime POST_NOTIFICATIONS request (Android 13+ denies-by-default
+  # until asked; the media notification is suppressed without it).
+  permission_handler: ^12.0.1
   google_fonts: ^8.1.0
   # 10.x conflicts with flutter_secure_storage 10.x on win32. Hold at 8.3.1
   # until either lib bumps win32 to 6.x.