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_ja.dart

@@ -370,4 +370,52 @@ class AppLocalizationsJa extends AppLocalizations {
 
   @override
   String get weekPlannedLabel => '週の計画済み';
+
+  @override
+  String get planMenuButton => '食事を計画する';
+
+  @override
+  String get planMenuTitle => '何を計画する？';
+
+  @override
+  String get planOptionSingleMeal => '1食';
+
+  @override
+  String get planOptionSingleMealDesc => '日と食事タイプを選択';
+
+  @override
+  String get planOptionDay => '1日';
+
+  @override
+  String get planOptionDayDesc => '1日分の全食事';
+
+  @override
+  String get planOptionDays => '数日';
+
+  @override
+  String get planOptionDaysDesc => '期間をカスタマイズ';
+
+  @override
+  String get planOptionWeek => '1週間';
+
+  @override
+  String get planOptionWeekDesc => '7日分まとめて';
+
+  @override
+  String get planSelectDate => '日付を選択';
+
+  @override
+  String get planSelectMealType => '食事タイプ';
+
+  @override
+  String get planSelectRange => '期間を選択';
+
+  @override
+  String get planGenerateButton => '計画する';
+
+  @override
+  String get planGenerating => 'プランを生成中…';
+
+  @override
+  String get planSuccess => 'メニューが計画されました！';
 }