feat: flexible meal planning wizard — plan 1 meal, 1 day, several days, or a week

Backend:
- migration 005: expand menu_items.meal_type CHECK to all 6 types (second_breakfast, afternoon_snack, snack)
- ai/types.go: add Days and MealTypes to MenuRequest for partial generation
- openai/menu.go: parametrize GenerateMenu — use requested meal types and day count; add caloric fractions for all 6 meal types
- menu/repository.go: add UpsertItemsInTx for partial upsert (preserves existing slots); fix meal_type sort order in GetByWeek
- menu/handler.go: add dates+meal_types path to POST /ai/generate-menu; extract fetchImages/saveRecipes helpers; returns {"plans":[...]} for dates mode; backward-compatible with week mode

Client:
- PlanMenuSheet: bottom sheet with 4 planning horizon options
- PlanDatePickerSheet: adaptive sheet with date strip (single day/meal) or custom CalendarRangePicker (multi-day/week); sliding 7-day window for week mode
- menu_service.dart: add generateForDates
- menu_provider.dart: add PlanMenuService (generates + invalidates week providers), lastPlannedDateProvider
- home_screen.dart: add _PlanMenuButton card below quick actions; opens planning wizard
- l10n: 16 new keys for planning UI across all 12 languages

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

client/lib/features/menu/menu_service.dart

@@ -26,6 +26,22 @@ class MenuService {
     return MenuPlan.fromJson(data);
   }
 
+  /// Generates meals for specific [dates] (YYYY-MM-DD) and [mealTypes].
+  /// Returns the updated MenuPlan for each affected week.
+  Future<List<MenuPlan>> generateForDates({
+    required List<String> dates,
+    required List<String> mealTypes,
+  }) async {
+    final data = await _client.post('/ai/generate-menu', data: {
+      'dates': dates,
+      'meal_types': mealTypes,
+    });
+    final plans = data['plans'] as List<dynamic>;
+    return plans
+        .map((planJson) => MenuPlan.fromJson(planJson as Map<String, dynamic>))
+        .toList();
+  }
+
   Future<void> updateMenuItem(String itemId, String recipeId) async {
     await _client.put('/menu/items/$itemId', data: {'recipe_id': recipeId});
   }