feat: improved receipt recognition, batch product add, and scan UX

- Rewrite receipt OCR prompt: completes truncated names, preserves fat%
  and flavour attributes, extracts weight/volume from line, infers
  typical package sizes for solid goods with quantity_confidence field
- Add quantity_confidence to RecognizedItem, EnrichedItem, and
  ProductJobResultItem; propagate through item enricher and worker
- Replace per-item create loop with single POST /user-products/batch call
  from RecognitionConfirmScreen
- Rebuild RecognitionConfirmScreen: amber qty border for low
  quantity_confidence, tappable product name → catalog picker,
  sort items by confidence, full L10n (no hardcoded strings)
- Add timestamps (HH:mm / d MMM HH:mm) to recent scan chips
- Show close-app hint on ProductJobWatchScreen (queued + processing)
- Refresh recentProductJobsProvider on watch screen init so new job
  appears without a manual pull-to-refresh
- App-level WidgetsBindingObserver refreshes product and dish job lists
  on resume, fixing stale lists after background/foreground transitions
- Add 9 new L10n keys across all 12 locales

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

backend/internal/domain/recognition/handler.go

@@ -103,13 +103,14 @@ type imagesRequest struct {
 
 // EnrichedItem is a recognized food item enriched with ingredient_mappings data.
 type EnrichedItem struct {
-	Name        string  `json:"name"`
-	Quantity    float64 `json:"quantity"`
-	Unit        string  `json:"unit"`
-	Category    string  `json:"category"`
-	Confidence  float64 `json:"confidence"`
-	MappingID   *string `json:"mapping_id"`
-	StorageDays int     `json:"storage_days"`
+	Name               string  `json:"name"`
+	Quantity           float64 `json:"quantity"`
+	Unit               string  `json:"unit"`
+	Category           string  `json:"category"`
+	Confidence         float64 `json:"confidence"`
+	QuantityConfidence float64 `json:"quantity_confidence"`
+	MappingID          *string `json:"mapping_id"`
+	StorageDays        int     `json:"storage_days"`
 }
 
 // ReceiptResponse is the response for POST /ai/recognize-receipt.