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>

backend/internal/adapters/ai/types.go

@@ -51,7 +51,7 @@ type NutritionInfo struct {
 	Approximate bool    `json:"approximate"`
 }
 
-// MenuRequest contains parameters for weekly menu generation.
+// MenuRequest contains parameters for menu generation.
 type MenuRequest struct {
 	UserGoal          string
 	DailyCalories     int
@@ -59,6 +59,12 @@ type MenuRequest struct {
 	CuisinePrefs      []string
 	AvailableProducts []string
 	Lang              string // ISO 639-1 target language code, e.g. "en", "ru"
+	// Days specifies which day-of-week slots (1=Monday…7=Sunday) to generate.
+	// When nil, all 7 days are generated.
+	Days []int
+	// MealTypes restricts generation to the listed meal types.
+	// When nil, defaults to ["breakfast","lunch","dinner"].
+	MealTypes []string
 }
 
 // DayPlan is the AI-generated plan for a single day.