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

assets/css/style.css

@@ -1969,6 +1969,46 @@ body.server-offline .bottom-nav {
     text-overflow: ellipsis;
 }
 
+/* — Scan status bar — */
+.scan-status-bar {
+    position: absolute;
+    bottom: 38px;
+    left: 0;
+    right: 0;
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    gap: 2px;
+    pointer-events: none;
+    z-index: 12;
+}
+.scan-status-method {
+    font-size: 0.58rem;
+    color: rgba(255,255,255,0.45);
+    text-transform: uppercase;
+    letter-spacing: 0.07em;
+    font-family: monospace;
+}
+.scan-status-msg {
+    font-size: 0.74rem;
+    color: rgba(255,255,255,0.9);
+    background: rgba(0,0,0,0.55);
+    padding: 3px 10px;
+    border-radius: 12px;
+    text-align: center;
+    max-width: 92%;
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    transition: color 0.2s, background 0.2s;
+    backdrop-filter: blur(3px);
+}
+.scan-status-msg:empty { visibility: hidden; }
+.scan-status-msg.state-partial { color: #fbbf24; }
+.scan-status-msg.state-invalid { color: #f87171; background: rgba(239,68,68,0.28); }
+.scan-status-msg.state-confirmed { color: #4ade80; background: rgba(74,222,128,0.22); }
+.scan-status-msg.state-retry { color: #fb923c; }
+
 /* — Viewport overlay controls (torch / zoom / flip) — */
 .scan-viewport-controls {
     position: absolute;