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/main.dart

@@ -0,0 +1,22 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:provider/provider.dart';
+import 'app.dart';
+import 'features/meal_planner/providers/meal_planner_provider.dart';
+import 'features/converter/providers/converter_provider.dart';
+import 'features/shopping_list/providers/shopping_list_provider.dart';
+
+void main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  await SystemChrome.setPreferredOrientations([DeviceOrientation.portraitUp]);
+  runApp(
+    MultiProvider(
+      providers: [
+        ChangeNotifierProvider(create: (_) => MealPlannerProvider()..load()),
+        ChangeNotifierProvider(create: (_) => ConverterProvider()..loadDb()),
+        ChangeNotifierProvider(create: (_) => ShoppingListProvider()..load()),
+      ],
+      child: const BitePlanApp(),
+    ),
+  );
+}