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_de.arb

@@ -135,5 +135,22 @@
   "generateWeekLabel": "Woche planen",
   "generateWeekSubtitle": "KI erstellt einen Menüplan mit Frühstück, Mittagessen und Abendessen für die ganze Woche",
   "generatingMenu": "Menü wird erstellt...",
-  "weekPlannedLabel": "Woche geplant"
+  "weekPlannedLabel": "Woche geplant",
+
+  "planMenuButton": "Mahlzeiten planen",
+  "planMenuTitle": "Was planen?",
+  "planOptionSingleMeal": "Einzelne Mahlzeit",
+  "planOptionSingleMealDesc": "Tag und Mahlzeittyp wählen",
+  "planOptionDay": "Ein Tag",
+  "planOptionDayDesc": "Alle Mahlzeiten für einen Tag",
+  "planOptionDays": "Mehrere Tage",
+  "planOptionDaysDesc": "Zeitraum anpassen",
+  "planOptionWeek": "Eine Woche",
+  "planOptionWeekDesc": "7 Tage auf einmal",
+  "planSelectDate": "Datum wählen",
+  "planSelectMealType": "Mahlzeittyp",
+  "planSelectRange": "Zeitraum wählen",
+  "planGenerateButton": "Planen",
+  "planGenerating": "Plan wird erstellt\u2026",
+  "planSuccess": "Menü geplant!"
 }