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/main.dart

@@ -1,19 +1,12 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'app.dart';
 
-void main() {
-  runApp(const App());
-}
-
-class App extends StatelessWidget {
-  const App({super.key});
-
-  @override
-  Widget build(BuildContext context) {
-    return MaterialApp(
-      title: 'Lumina',
-      debugShowCheckedModeBanner: false,
-      theme: ThemeData(useMaterial3: true),
-      home: const Scaffold(body: Center(child: Text('Lumina'))),
-    );
-  }
+void main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  runApp(
+    const ProviderScope(
+      child: App(),
+    ),
+  );
 }