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/activity_provider.dart

@@ -50,7 +50,7 @@ class ActivityNotifier extends Notifier<AsyncValue<void>> {
     required String colourHex,
   }) async {
     state = const AsyncLoading();
-    state = await AsyncValue.guard(
+    final next = await AsyncValue.guard(
       () => ref.read(activityDaoProvider).insertActivity(
             ActivitiesCompanion.insert(
               name: name,
@@ -60,13 +60,17 @@ class ActivityNotifier extends Notifier<AsyncValue<void>> {
             ),
           ),
     );
+    if (!ref.mounted) return;
+    state = next;
   }
 
   Future<void> deleteActivity(int id) async {
     state = const AsyncLoading();
-    state = await AsyncValue.guard(
+    final next = await AsyncValue.guard(
       () => ref.read(activityDaoProvider).deleteActivity(id),
     );
+    if (!ref.mounted) return;
+    state = next;
   }
 }