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

index.html

@@ -251,6 +251,11 @@
                 </div>
                 <!-- Live partial code preview -->
                 <div class="scan-live-code" id="scan-live-code" style="display:none"></div>
+                <!-- Scan status bar -->
+                <div class="scan-status-bar" id="scan-status-bar">
+                    <span id="scan-status-method" class="scan-status-method"></span>
+                    <span id="scan-status-msg" class="scan-status-msg" data-i18n="scan.status_ready"></span>
+                </div>
                 <!-- Success flash overlay -->
                 <div class="scan-confirm-overlay" id="scan-confirm-overlay" style="display:none">
                     <div class="scan-confirm-check">✓</div>