feat: barcode scan visual feedback + EAN checksum validation

- Add scan-status-bar overlay inside scanner viewport showing:
  - Active scan method (Native API / Quagga / Native + Quagga)
  - Scanning state: ready, scanning, partial read, invalid, confirmed
- Validate EAN-8/EAN-13/UPC checksums in Quagga path before confirming
  (native BarcodeDetector validates internally; Quagga can return false positives)
- Show 'invalid barcode, retrying' message with invalid code highlighted
- Reset invalid barcode confidence counter on invalid read so scanner retries
- Spawn parallel Quagga scan with 'combined scan active' status message
- Add 6 translation keys (scan.status_*) in all 5 language files

translations/es.json

@@ -211,7 +211,13 @@
         "barcode_acquired": "🔖 Código de barras escaneado: {code}",
         "scan_barcode": "🔖 Escanear código de barras",
         "create_named": "Crear {name}",
-        "new_without_barcode": "Nuevo producto sin código de barras"
+        "new_without_barcode": "Nuevo producto sin código de barras",
+        "status_ready": "Apunta la cámara al código de barras",
+        "status_scanning": "Escaneando...",
+        "status_partial": "Detectado: {code} — verificando...",
+        "status_invalid": "Inválido: {code} — reintentando",
+        "status_confirmed": "Confirmado!",
+        "status_parallel": "Escaneo combinado activo..."
     },
     "action": {
         "title": "¿Qué quieres hacer?",