feat: implement Iteration 3 — product/receipt/dish recognition

Backend:
- gemini/client.go: refactor to shared callGroq transport; add
  generateVisionContent using llama-3.2-11b-vision-preview model
- gemini/recognition.go: RecognizeReceipt, RecognizeProducts,
  RecognizeDish (vision), ClassifyIngredient (text); shared parseJSON helper
- ingredient/repository.go: add FuzzyMatch (wraps Search, returns best hit)
- recognition/handler.go: POST /ai/recognize-receipt, /ai/recognize-products,
  /ai/recognize-dish; enrichItems with fuzzy match + AI classify fallback;
  parallel multi-image processing with deduplication
- server.go + main.go: wire recognition handler under /ai routes

Flutter:
- pubspec.yaml: add image_picker ^1.1.0
- AndroidManifest.xml: add CAMERA and READ_EXTERNAL_STORAGE permissions
- Info.plist: add NSCameraUsageDescription and NSPhotoLibraryUsageDescription
- recognition_service.dart: RecognitionService wrapping /ai/* endpoints;
  RecognizedItem, ReceiptResult, DishResult models
- scan_screen.dart: mode selector (receipt / products / dish / manual);
  image source picker; loading overlay; navigates to confirm or dish screen
- recognition_confirm_screen.dart: editable list of recognized items;
  inline qty/unit editing; swipe-to-delete; batch-add to pantry
- dish_result_screen.dart: dish name, KBZHU breakdown, similar dishes chips
- app_router.dart: /scan, /scan/confirm, /scan/dish routes (no bottom nav)
- products_screen.dart: FAB now shows bottom sheet with Manual / Scan options

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

backend/internal/ingredient/repository.go

@@ -94,6 +94,19 @@ func (r *Repository) GetByID(ctx context.Context, id string) (*IngredientMapping
 	return m, err
 }
 
+// FuzzyMatch finds the single best matching ingredient mapping for a given name.
+// Returns nil, nil when no match is found.
+func (r *Repository) FuzzyMatch(ctx context.Context, name string) (*IngredientMapping, error) {
+	results, err := r.Search(ctx, name, 1)
+	if err != nil {
+		return nil, err
+	}
+	if len(results) == 0 {
+		return nil, nil
+	}
+	return results[0], nil
+}
+
 // Search finds ingredient mappings matching the query string.
 // Uses a three-level strategy: exact aliases match, ILIKE, and pg_trgm similarity.
 func (r *Repository) Search(ctx context.Context, query string, limit int) ([]*IngredientMapping, error) {