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/core/constants/app_constants.dart

@@ -0,0 +1,30 @@
+const List<String> kDayIds = [
+  'lunedi',
+  'martedi',
+  'mercoledi',
+  'giovedi',
+  'venerdi',
+  'sabato',
+  'domenica',
+];
+
+const Map<String, String> kDayLabels = {
+  'lunedi': 'Lunedì',
+  'martedi': 'Martedì',
+  'mercoledi': 'Mercoledì',
+  'giovedi': 'Giovedì',
+  'venerdi': 'Venerdì',
+  'sabato': 'Sabato',
+  'domenica': 'Domenica',
+};
+
+const List<String> kMealSlots = ['colazione', 'pranzo', 'cena'];
+
+const Map<String, String> kMealSlotLabels = {
+  'colazione': 'Colazione',
+  'pranzo': 'Pranzo',
+  'cena': 'Cena',
+};
+
+const String kStorageKeyMeals = 'meals';
+const String kStorageKeyShopping = 'shopping_list';

lib/core/theme/app_theme.dart

@@ -0,0 +1,24 @@
+import 'package:flutter/material.dart';
+
+class AppTheme {
+  static const _seed = Color(0xFF2d6a4f);
+
+  static ThemeData light() {
+    final scheme = ColorScheme.fromSeed(seedColor: _seed);
+    return ThemeData(
+      useMaterial3: true,
+      colorScheme: scheme,
+      cardTheme: CardThemeData(
+        elevation: 0,
+        shape: RoundedRectangleBorder(
+          borderRadius: BorderRadius.circular(12),
+          side: BorderSide(color: scheme.outlineVariant),
+        ),
+      ),
+      inputDecorationTheme: InputDecorationTheme(
+        border: OutlineInputBorder(borderRadius: BorderRadius.circular(10)),
+        contentPadding: const EdgeInsets.symmetric(horizontal: 14, vertical: 12),
+      ),
+    );
+  }
+}