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/app.dart

@@ -0,0 +1,44 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'core/providers/settings_provider.dart';
+import 'features/log_mood/log_mood_screen.dart';
+
+class App extends ConsumerWidget {
+  const App({super.key});
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final onboardingAsync = ref.watch(isOnboardingCompleteProvider);
+
+    return MaterialApp(
+      title: 'DailyYou',
+      debugShowCheckedModeBanner: false,
+      theme: _lightTheme(),
+      darkTheme: _darkTheme(),
+      themeMode: ThemeMode.system,
+      home: onboardingAsync.when(
+        data: (complete) => const LogMoodScreen(),
+        loading: () => const Scaffold(
+          body: Center(child: CircularProgressIndicator()),
+        ),
+        error: (e, _) => const LogMoodScreen(),
+      ),
+    );
+  }
+
+  ThemeData _lightTheme() => ThemeData(
+        useMaterial3: true,
+        colorScheme: ColorScheme.fromSeed(
+          seedColor: const Color(0xFF6B8CFF),
+          brightness: Brightness.light,
+        ),
+      );
+
+  ThemeData _darkTheme() => ThemeData(
+        useMaterial3: true,
+        colorScheme: ColorScheme.fromSeed(
+          seedColor: const Color(0xFF6B8CFF),
+          brightness: Brightness.dark,
+        ),
+      );
+}