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/features/menu/menu_provider.dart

@@ -151,3 +151,54 @@ final diaryProvider =
     StateNotifierProvider.family<DiaryNotifier, AsyncValue<List<DiaryEntry>>, String>(
   (ref, date) => DiaryNotifier(ref.read(menuServiceProvider), date),
 );
+
+// ── Plan menu helpers ─────────────────────────────────────────
+
+/// The latest future date (among cached menus for the current and next two weeks)
+/// that already has at least one planned meal slot. Returns null when no future
+/// meals are planned.
+final lastPlannedDateProvider = Provider<DateTime?>((ref) {
+  final today = DateTime.now();
+  DateTime? latestDate;
+  for (var offsetDays = 0; offsetDays <= 14; offsetDays += 7) {
+    final weekDate = today.add(Duration(days: offsetDays));
+    final plan = ref.watch(menuProvider(isoWeekString(weekDate))).valueOrNull;
+    if (plan == null) continue;
+    for (final day in plan.days) {
+      final dayDate = DateTime.parse(day.date);
+      if (dayDate.isAfter(today) && day.meals.isNotEmpty) {
+        if (latestDate == null || dayDate.isAfter(latestDate)) {
+          latestDate = dayDate;
+        }
+      }
+    }
+  }
+  return latestDate;
+});
+
+/// Service for planning meals across specific dates.
+/// Calls the API and then invalidates the affected week providers so that
+/// [plannedMealsProvider] picks up the new slots automatically.
+class PlanMenuService {
+  final Ref _ref;
+
+  const PlanMenuService(this._ref);
+
+  Future<void> generateForDates({
+    required List<String> dates,
+    required List<String> mealTypes,
+  }) async {
+    final menuService = _ref.read(menuServiceProvider);
+    final plans = await menuService.generateForDates(
+      dates: dates,
+      mealTypes: mealTypes,
+    );
+    for (final plan in plans) {
+      _ref.invalidate(menuProvider(isoWeekString(DateTime.parse(plan.weekStart))));
+    }
+  }
+}
+
+final planMenuServiceProvider = Provider<PlanMenuService>((ref) {
+  return PlanMenuService(ref);
+});