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/features/shopping_list/models/shopping_item.dart

@@ -0,0 +1,30 @@
+class ShoppingItem {
+  final String id;
+  final String name;
+  final bool checked;
+
+  const ShoppingItem({
+    required this.id,
+    required this.name,
+    this.checked = false,
+  });
+
+  ShoppingItem copyWith({String? id, String? name, bool? checked}) =>
+      ShoppingItem(
+        id: id ?? this.id,
+        name: name ?? this.name,
+        checked: checked ?? this.checked,
+      );
+
+  Map<String, dynamic> toJson() => {
+        'id': id,
+        'name': name,
+        'checked': checked,
+      };
+
+  factory ShoppingItem.fromJson(Map<String, dynamic> json) => ShoppingItem(
+        id: json['id'] as String,
+        name: json['name'] as String,
+        checked: json['checked'] as bool? ?? false,
+      );
+}