fix: ZXing per-frame 'no detection' errors were being treated as camera failures

ZXing's decodeFromVideoDevice fires its callback on every video frame
(~30fps). When no barcode is in view, it passes `(null, err)` where err
is 'No MultiFormat Readers were able to detect the code' — this is
NORMAL, not a hardware failure.

The error branch now filters out 'multiformat' and 'unable to detect'
messages, only calling handleCameraError for real device-level issues
(permission denied, camera not found, stream broken).

static/index.html

@@ -2909,7 +2909,11 @@
           }
         }, 5000);
 
-        // decodeFromVideoDevice handles camera + scanning in one call
+        // decodeFromVideoDevice handles camera + scanning in one call.
+        // The callback fires on every video frame (~30fps). ZXing passes
+        // "No MultiFormat Readers..." as an error when no barcode is in view
+        // — that's NORMAL, not a hardware failure. Only treat real device
+        // errors (permission denied, camera not found, stream broken) as fatal.
         codeReader.decodeFromVideoDevice(deviceId, video, (result, err) => {
           // Clear the timeout once we get any callback (stream started or error)
           clearTimeout(cameraFailTimer);
@@ -2919,9 +2923,12 @@
               handleBarcode(val.trim());
             }
           } else if (err && scanningActive) {
-            // ZXing reports camera errors through the callback (not as exceptions)
-            console.error('ZXing camera error:', err);
-            handleCameraError(new Error(err.message || err));
+            // Filter out "no barcode in frame" — that's normal, not a failure
+            const msg = (err.message || err || '').toLowerCase();
+            if (!msg.includes('multiformat') && !msg.includes('unable to detect')) {
+              console.error('ZXing camera error:', err);
+              handleCameraError(new Error(err.message || err));
+            }
           }
         });
       } catch (e) {