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/shared/widgets/empty_state.dart

@@ -0,0 +1,48 @@
+import 'package:flutter/material.dart';
+
+class EmptyState extends StatelessWidget {
+  final IconData icon;
+  final String title;
+  final String? subtitle;
+
+  const EmptyState({
+    super.key,
+    required this.icon,
+    required this.title,
+    this.subtitle,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final muted = theme.colorScheme.onSurface.withValues(alpha: 0.4);
+
+    return Center(
+      child: Padding(
+        padding: const EdgeInsets.all(40),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Icon(icon, size: 48, color: muted),
+            const SizedBox(height: 16),
+            Text(
+              title,
+              style: theme.textTheme.bodyLarge?.copyWith(color: muted),
+              textAlign: TextAlign.center,
+            ),
+            if (subtitle != null) ...[
+              const SizedBox(height: 6),
+              Text(
+                subtitle!,
+                style: theme.textTheme.bodySmall?.copyWith(
+                  color: theme.colorScheme.onSurface.withValues(alpha: 0.3),
+                ),
+                textAlign: TextAlign.center,
+              ),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}