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_zh.arb

@@ -172,5 +172,32 @@
   "jobTypeReceipt": "收据",
   "jobTypeProducts": "产品",
   "scanSubmitting": "提交中...",
-  "processingProducts": "处理中..."
+  "processingProducts": "处理中...",
+  "clearAllProducts": "清空列表",
+  "clearAllConfirmTitle": "清空所有产品？",
+  "clearAllConfirmMessage": "所有产品将被永久删除。",
+  "addManually": "手动",
+  "scan": "扫描",
+  "addProduct": "添加",
+  "searchProducts": "搜索产品",
+  "searchProductsHint": "输入产品名称搜索或手动添加",
+  "noSearchResults": "未找到{query}的结果",
+  "@noSearchResults": {
+    "placeholders": {
+      "query": {
+        "type": "String"
+      }
+    }
+  },
+  "quantity": "数量",
+  "storageDays": "保存天数",
+  "addToShelf": "添加到储藏室",
+  "errorGeneric": "出错了",
+  "nutritionOptional": "每100克营养成分（可选）",
+  "calories": "卡路里",
+  "protein": "蛋白质",
+  "fat": "脂肪",
+  "carbs": "碳水化合物",
+  "fiber": "膳食纤维",
+  "productAddedToShelf": "已添加到储藏室"
 }