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/product_worker.go

@@ -127,13 +127,14 @@ func (pool *ProductWorkerPool) processJob(workerContext context.Context, jobID s
 	resultItems := make([]ProductJobResultItem, len(enriched))
 	for index, item := range enriched {
 		resultItems[index] = ProductJobResultItem{
-			Name:        item.Name,
-			Quantity:    item.Quantity,
-			Unit:        item.Unit,
-			Category:    item.Category,
-			Confidence:  item.Confidence,
-			MappingID:   item.MappingID,
-			StorageDays: item.StorageDays,
+			Name:               item.Name,
+			Quantity:           item.Quantity,
+			Unit:               item.Unit,
+			Category:           item.Category,
+			Confidence:         item.Confidence,
+			QuantityConfidence: item.QuantityConfidence,
+			MappingID:          item.MappingID,
+			StorageDays:        item.StorageDays,
 		}
 	}