feat: wire up app shell, log mood screen, and fix Android build

- Bootstrap ProviderScope in main.dart and move App to app.dart with light/dark theming
- Add initial LogMoodScreen with mood entry flow and shared activity/mood widgets
- Add kIsWeb guard in app_database.dart for drift web support
- Enable Android core library desugaring required by flutter_local_notifications 21.x
- Guard all async notifier state assignments with ref.mounted checks to prevent
  use-after-dispose errors when providers are auto-disposed mid-operation

lib/core/providers/llm_provider.dart

@@ -33,11 +33,12 @@ class LlmResponseNotifier extends Notifier<AsyncValue<String>> {
 
     try {
       await for (final token in service.generateStream(prompt)) {
-        if (_cancelled) break;
+        if (_cancelled || !ref.mounted) break;
         buffer.write(token);
         state = AsyncData(buffer.toString());
       }
     } catch (e, st) {
+      if (!ref.mounted) return;
       state = AsyncError(e, st);
     }
   }