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/en.json

@@ -214,7 +214,13 @@
         "scan_barcode": "🔖 Scan Barcode",
         "create_named": "Create {name}",
         "new_without_barcode": "New product without barcode",
-        "stock_in_pantry": "Already in pantry:"
+        "stock_in_pantry": "Already in pantry:",
+        "status_ready": "Point camera at barcode",
+        "status_scanning": "Scanning...",
+        "status_partial": "Detected: {code} — verifying...",
+        "status_invalid": "Invalid: {code} — retrying",
+        "status_confirmed": "Confirmed!",
+        "status_parallel": "Using combined scan methods..."
     },
     "action": {
         "title": "What do you want to do?",