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_it.arb

@@ -172,5 +172,32 @@
   "jobTypeReceipt": "Scontrino",
   "jobTypeProducts": "Prodotti",
   "scanSubmitting": "Invio...",
-  "processingProducts": "Elaborazione..."
+  "processingProducts": "Elaborazione...",
+  "clearAllProducts": "Cancella tutto",
+  "clearAllConfirmTitle": "Cancellare tutti i prodotti?",
+  "clearAllConfirmMessage": "Tutti i prodotti verranno eliminati definitivamente.",
+  "addManually": "Manualmente",
+  "scan": "Scansiona",
+  "addProduct": "Aggiungi",
+  "searchProducts": "Cerca prodotti",
+  "searchProductsHint": "Digita il nome del prodotto o aggiungi manualmente",
+  "noSearchResults": "Nessun risultato per \"{query}\"",
+  "@noSearchResults": {
+    "placeholders": {
+      "query": {
+        "type": "String"
+      }
+    }
+  },
+  "quantity": "Quantità",
+  "storageDays": "Giorni di conservazione",
+  "addToShelf": "Aggiungi alla dispensa",
+  "errorGeneric": "Qualcosa è andato storto",
+  "nutritionOptional": "Valori nutrizionali per 100g (opzionale)",
+  "calories": "Calorie",
+  "protein": "Proteine",
+  "fat": "Grassi",
+  "carbs": "Carboidrati",
+  "fiber": "Fibre",
+  "productAddedToShelf": "Aggiunto alla dispensa"
 }