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/l10n/app_localizations_zh.dart

@@ -370,4 +370,52 @@ class AppLocalizationsZh extends AppLocalizations {
 
   @override
   String get weekPlannedLabel => '本周已规划';
+
+  @override
+  String get planMenuButton => '规划餐食';
+
+  @override
+  String get planMenuTitle => '规划什么？';
+
+  @override
+  String get planOptionSingleMeal => '单次餐食';
+
+  @override
+  String get planOptionSingleMealDesc => '选择日期和餐食类型';
+
+  @override
+  String get planOptionDay => '一天';
+
+  @override
+  String get planOptionDayDesc => '一天的所有餐食';
+
+  @override
+  String get planOptionDays => '几天';
+
+  @override
+  String get planOptionDaysDesc => '自定义日期范围';
+
+  @override
+  String get planOptionWeek => '一周';
+
+  @override
+  String get planOptionWeekDesc => '一次规划7天';
+
+  @override
+  String get planSelectDate => '选择日期';
+
+  @override
+  String get planSelectMealType => '餐食类型';
+
+  @override
+  String get planSelectRange => '选择时间段';
+
+  @override
+  String get planGenerateButton => '规划';
+
+  @override
+  String get planGenerating => '正在生成计划…';
+
+  @override
+  String get planSuccess => '菜单已规划！';
 }