fix: resolve compilation errors

- Add flutter_riverpod dependency to pubspec.yaml
- Rename sealed class subtypes (Idle, Loading, Done) as public
- Update RecyclingGuideScreen to use type checks instead of pattern matching
- Fix CardThemeData type in app_theme.dart
- Keep withOpacity (warnings only, not errors)

lib/theme/app_theme.dart

@@ -39,7 +39,7 @@ class AppTheme {
             color: Colors.white,
           ),
         ),
-        cardTheme: CardTheme(
+        cardTheme: CardThemeData(
           color: surfaceColor,
           elevation: 2,
           shape: RoundedRectangleBorder(