feat: product search screen with catalog add and success feedback

- Add product search screen (/products/search) as primary add flow;
  "Add" button on products list opens search, manual entry remains as fallback
- Add to shelf bottom sheet with AnimatedSwitcher success view (green checkmark)
  and SnackBar confirmation on the search screen via onAdded callback
- Manual add (AddProductScreen) shows SnackBar on success before popping back
- Extend AddProductScreen with optional nutrition fields (calories, protein,
  fat, carbs, fiber); auto-fills from catalog selection and auto-expands section
- Auto-upsert catalog product on backend when nutrition data is provided
  without a primary_product_id, linking the user product to the catalog
- Add fiber_per_100g field to CatalogProduct model and CreateRequest
- Add 16 new L10n keys across all 12 locales (addProduct, addManually,
  searchProducts, quantity, storageDays, addToShelf, nutritionOptional,
  calories, protein, fat, carbs, fiber, productAddedToShelf, etc.)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/domain/userproduct/entity.go

@@ -28,6 +28,14 @@ type CreateRequest struct {
 	Unit             string   `json:"unit"`
 	Category         *string  `json:"category"`
 	StorageDays      int      `json:"storage_days"`
+	// Optional nutrition fields per 100g.
+	// When provided and primary_product_id is absent, the backend upserts a catalog
+	// product and links it automatically.
+	CaloriesPer100g *float64 `json:"calories_per_100g"`
+	ProteinPer100g  *float64 `json:"protein_per_100g"`
+	FatPer100g      *float64 `json:"fat_per_100g"`
+	CarbsPer100g    *float64 `json:"carbs_per_100g"`
+	FiberPer100g    *float64 `json:"fiber_per_100g"`
 }
 
 // UpdateRequest is the body for PUT /user-products/{id}.