fix: barcode scan rate 50ms→300ms — decoder was thrashing, missed barcodes

delayBetweenScanAttempts was set to 50ms (20 scans/sec) but with
TRY_HARDER enabled and 13 barcode formats configured, the ZXing decoder
couldn't keep up — frames were skipped and barcodes visible in the
viewfinder were never detected. 300ms gives each frame enough processing
time while still feeling real-time.

static/index.html

@@ -2851,7 +2851,9 @@
 
     function _barcodeOptions() {
       return {
-        delayBetweenScanAttempts: 50,   // scan every 50ms (was default ~500ms)
+        delayBetweenScanAttempts: 300,  // scan every 300ms (was 50ms — too fast
+                                        // for TRY_HARDER with 13 formats, caused
+                                        // decoder thrashing and missed barcodes)
         delayBetweenScanSuccess: 2000,  // 2s cooldown after a successful scan
       };
     }