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>

client/lib/l10n/app_en.arb

@@ -172,5 +172,32 @@
   "jobTypeReceipt": "Receipt",
   "jobTypeProducts": "Products",
   "scanSubmitting": "Submitting...",
-  "processingProducts": "Processing..."
+  "processingProducts": "Processing...",
+  "clearAllProducts": "Clear all",
+  "clearAllConfirmTitle": "Clear all products?",
+  "clearAllConfirmMessage": "All products will be permanently deleted.",
+  "addManually": "Manually",
+  "scan": "Scan",
+  "addProduct": "Add",
+  "searchProducts": "Search products",
+  "searchProductsHint": "Type a product name to search or add manually",
+  "noSearchResults": "No results for \"{query}\"",
+  "@noSearchResults": {
+    "placeholders": {
+      "query": {
+        "type": "String"
+      }
+    }
+  },
+  "quantity": "Quantity",
+  "storageDays": "Storage days",
+  "addToShelf": "Add to pantry",
+  "errorGeneric": "Something went wrong",
+  "nutritionOptional": "Nutrition per 100g (optional)",
+  "calories": "Calories",
+  "protein": "Protein",
+  "fat": "Fat",
+  "carbs": "Carbohydrates",
+  "fiber": "Fiber",
+  "productAddedToShelf": "Added to pantry"
 }