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_zh.arb

@@ -135,5 +135,22 @@
   "generateWeekLabel": "规划本周",
   "generateWeekSubtitle": "AI将为整周创建含早餐、午餐和晚餐的菜单",
   "generatingMenu": "正在生成菜单...",
-  "weekPlannedLabel": "本周已规划"
+  "weekPlannedLabel": "本周已规划",
+
+  "planMenuButton": "规划餐食",
+  "planMenuTitle": "规划什么？",
+  "planOptionSingleMeal": "单次餐食",
+  "planOptionSingleMealDesc": "选择日期和餐食类型",
+  "planOptionDay": "一天",
+  "planOptionDayDesc": "一天的所有餐食",
+  "planOptionDays": "几天",
+  "planOptionDaysDesc": "自定义日期范围",
+  "planOptionWeek": "一周",
+  "planOptionWeekDesc": "一次规划7天",
+  "planSelectDate": "选择日期",
+  "planSelectMealType": "餐食类型",
+  "planSelectRange": "选择时间段",
+  "planGenerateButton": "规划",
+  "planGenerating": "正在生成计划\u2026",
+  "planSuccess": "菜单已规划！"
 }