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_de.dart

@@ -374,4 +374,52 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get weekPlannedLabel => 'Woche geplant';
+
+  @override
+  String get planMenuButton => 'Mahlzeiten planen';
+
+  @override
+  String get planMenuTitle => 'Was planen?';
+
+  @override
+  String get planOptionSingleMeal => 'Einzelne Mahlzeit';
+
+  @override
+  String get planOptionSingleMealDesc => 'Tag und Mahlzeittyp wählen';
+
+  @override
+  String get planOptionDay => 'Ein Tag';
+
+  @override
+  String get planOptionDayDesc => 'Alle Mahlzeiten für einen Tag';
+
+  @override
+  String get planOptionDays => 'Mehrere Tage';
+
+  @override
+  String get planOptionDaysDesc => 'Zeitraum anpassen';
+
+  @override
+  String get planOptionWeek => 'Eine Woche';
+
+  @override
+  String get planOptionWeekDesc => '7 Tage auf einmal';
+
+  @override
+  String get planSelectDate => 'Datum wählen';
+
+  @override
+  String get planSelectMealType => 'Mahlzeittyp';
+
+  @override
+  String get planSelectRange => 'Zeitraum wählen';
+
+  @override
+  String get planGenerateButton => 'Planen';
+
+  @override
+  String get planGenerating => 'Plan wird erstellt…';
+
+  @override
+  String get planSuccess => 'Menü geplant!';
 }