feat: riscrivi app in Flutter

Riscrittura completa da Vue 3 + Capacitor a Flutter/Dart.

Architettura feature-based (lib/features/):
- meal_planner: modello MealPlan/DayPlan, provider, pagina e widget
- converter: modello ConversionEntry, provider con ricerca, pagina
- shopping_list: modello ShoppingItem, provider con deduplicazione, pagina e widget

Core:
- Material 3 con seed color #2d6a4f
- NavigationBar + IndexedStack (preserva stato dei tab)
- Portrait lock via SystemChrome
- Persistenza con shared_preferences
- 50+ alimenti × metodi cottura in assets/data/conversions.json

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/shared/services/storage_service.dart

@@ -0,0 +1,13 @@
+import 'package:shared_preferences/shared_preferences.dart';
+
+class StorageService {
+  static Future<String?> load(String key) async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getString(key);
+  }
+
+  static Future<void> save(String key, String value) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(key, value);
+  }
+}