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>

pubspec.yaml

@@ -0,0 +1,35 @@
+name: biteplan
+description: App per meal planning, conversione crudo/cotto e lista della spesa.
+publish_to: 'none'
+version: 2.0.0+1
+
+environment:
+  sdk: '>=3.3.0 <4.0.0'
+
+dependencies:
+  flutter:
+    sdk: flutter
+  provider: ^6.1.2
+  shared_preferences: ^2.3.2
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  integration_test:
+    sdk: flutter
+  flutter_lints: ^4.0.0
+  flutter_launcher_icons: ^0.14.1
+
+flutter_launcher_icons:
+  android: true
+  ios: false
+  image_path: "assets/icon-only.png"
+  min_sdk_android: 21
+  adaptive_icon_background: "#2d6a4f"
+  adaptive_icon_foreground: "assets/icon-only.png"
+
+flutter:
+  uses-material-design: true
+  assets:
+    - assets/data/conversions.json
+    - assets/icon-only.png