feat: implement Iteration 4 — menu planning, shopping list, diary

Backend:
- Migrations 007 (menu_plans, menu_items, shopping_lists) and
  008 (meal_diary)
- gemini/menu.go: GenerateMenu — 7-day × 3-meal plan via one Groq call
- internal/menu: model, repository (GetByWeek, SaveMenuInTx,
  shopping list CRUD), handler (GET/PUT/DELETE /menu,
  POST /ai/generate-menu, shopping list endpoints)
- internal/diary: model, repository, handler (GET/POST/DELETE /diary)
- Increase server WriteTimeout to 120s for long AI calls
- api_client.go: add patch() and postList() helpers

Flutter:
- shared/models: menu.dart, shopping_item.dart, diary_entry.dart
- features/menu: menu_service.dart, menu_provider.dart
  (MenuNotifier, ShoppingListNotifier, DiaryNotifier with family)
- MenuScreen: 7-day view, week nav, skeleton on generation,
  generate FAB with confirmation dialog
- ShoppingListScreen: items by category, optimistic checkbox toggle
- DiaryScreen: daily entries with swipe-to-delete, add-entry sheet
- Router: /menu/shopping-list and /menu/diary routes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/shared/models/shopping_item.dart

@@ -0,0 +1,46 @@
+class ShoppingItem {
+  final String name;
+  final String category;
+  final double amount;
+  final String unit;
+  final bool checked;
+  final double inStock;
+
+  const ShoppingItem({
+    required this.name,
+    required this.category,
+    required this.amount,
+    required this.unit,
+    required this.checked,
+    required this.inStock,
+  });
+
+  factory ShoppingItem.fromJson(Map<String, dynamic> json) {
+    return ShoppingItem(
+      name: json['name'] as String? ?? '',
+      category: json['category'] as String? ?? 'other',
+      amount: (json['amount'] as num?)?.toDouble() ?? 0,
+      unit: json['unit'] as String? ?? '',
+      checked: json['checked'] as bool? ?? false,
+      inStock: (json['in_stock'] as num?)?.toDouble() ?? 0,
+    );
+  }
+
+  Map<String, dynamic> toJson() => {
+        'name': name,
+        'category': category,
+        'amount': amount,
+        'unit': unit,
+        'checked': checked,
+        'in_stock': inStock,
+      };
+
+  ShoppingItem copyWith({bool? checked}) => ShoppingItem(
+        name: name,
+        category: category,
+        amount: amount,
+        unit: unit,
+        checked: checked ?? this.checked,
+        inStock: inStock,
+      );
+}