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/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),
+      ),
+    );
+  }
+}