perf: throttle jsQR camera fallback to a few decodes per second

The TOTP QR reader relied solely on window.BarcodeDetector. On desktop
Chrome/Edge (Windows/Linux) that interface exists but has no working
backend, so detect() returns an empty array: uploading a valid QR image
fell through to "no QR code found" and the camera path bailed to
"unsupported" with an empty preview.

Add a dependency-free jsQR canvas fallback. decodeTotpQrImage now tries
BarcodeDetector first when present, then decodes the image via jsQR
before reporting not-found. The camera reader no longer hard-returns
"unsupported" when only BarcodeDetector is missing: it starts the camera
whenever getUserMedia is available and decodes frames with jsQR, which
also lets the preview render.

Fixes #276

package-lock.json

@@ -14,6 +14,7 @@
         "@tanstack/react-query": "^5.101.2",
         "@zip.js/zip.js": "^2.8.26",
         "fflate": "^0.8.3",
+        "jsqr": "1.4.0",
         "lucide-preact": "^1.22.0",
         "preact": "^10.29.3",
         "qrcode-generator": "^2.0.4",
@@ -3442,6 +3443,12 @@
         "node": ">=6"
       }
     },
+    "node_modules/jsqr": {
+      "version": "1.4.0",
+      "resolved": "https://registry.npmjs.org/jsqr/-/jsqr-1.4.0.tgz",
+      "integrity": "sha512-dxLob7q65Xg2DvstYkRpkYtmKm2sPJ9oFhrhmudT1dZvNFFTlroai3AWSpLey/w5vMcLBXRgOJsbXpdN9HzU/A==",
+      "license": "Apache-2.0"
+    },
     "node_modules/kleur": {
       "version": "4.1.5",
       "resolved": "https://registry.npmjs.org/kleur/-/kleur-4.1.5.tgz",

package.json

@@ -67,6 +67,7 @@
     "@tanstack/react-query": "^5.101.2",
     "@zip.js/zip.js": "^2.8.26",
     "fflate": "^0.8.3",
+    "jsqr": "1.4.0",
     "lucide-preact": "^1.22.0",
     "preact": "^10.29.3",
     "qrcode-generator": "^2.0.4",

webapp/src/components/vault/VaultEditor.tsx

@@ -1,6 +1,7 @@
 import type { RefObject } from 'preact';
 import { createPortal } from 'preact/compat';
 import { ArrowDown, ArrowUp, CheckCheck, Download, Paperclip, Plus, QrCode, RefreshCw, Star, StarOff, Trash2, Upload, X } from 'lucide-preact';
+import jsQR from 'jsqr';
 import { useEffect, useRef, useState } from 'preact/hooks';
 import { useDialogLifecycle } from '@/components/ConfirmDialog';
 import type { Cipher, Folder, VaultDraft, VaultDraftField } from '@/lib/types';
@@ -171,16 +172,38 @@ export default function VaultEditor(props: VaultEditorProps) {
     return new window.BarcodeDetector({ formats: ['qr_code'] });
   };
 
-  const decodeTotpQrImage = async (source: ImageBitmapSource): Promise<boolean> => {
+  const decodeTotpQrCanvas = (source: ImageBitmap | HTMLVideoElement): string => {
+    const width = 'videoWidth' in source ? source.videoWidth : source.width;
+    const height = 'videoHeight' in source ? source.videoHeight : source.height;
+    if (!width || !height) return '';
+    const canvas = document.createElement('canvas');
+    canvas.width = width;
+    canvas.height = height;
+    const context = canvas.getContext('2d');
+    if (!context) return '';
+    // jsQR ignores alpha and reads RGB directly, so transparent pixels would be
+    // treated as black. Composite over white first so transparent-background QR
+    // exports do not become black-on-black and fail to decode.
+    context.fillStyle = '#ffffff';
+    context.fillRect(0, 0, width, height);
+    context.drawImage(source, 0, 0, width, height);
+    const imageData = context.getImageData(0, 0, width, height);
+    return String(jsQR(imageData.data, width, height)?.data || '').trim();
+  };
+
+  const decodeTotpQrImage = async (source: ImageBitmap): Promise<boolean> => {
     const detector = createTotpQrDetector();
-    if (!detector) {
-      setTotpQrStatus(t('txt_totp_qr_unsupported'));
-      return false;
-    }
+    if (detector) {
+      try {
         const results = await detector.detect(source);
         const value = String(results[0]?.rawValue || '').trim();
-    if (!value) return false;
-    return applyTotpQrValue(value);
+        if (value && applyTotpQrValue(value)) return true;
+      } catch {
+        // Fall back to jsQR when the native detector is present but not usable.
+      }
+    }
+    const value = decodeTotpQrCanvas(source);
+    return value ? applyTotpQrValue(value) : false;
   };
 
   const handleTotpQrFile = async (file: File | null) => {
@@ -206,14 +229,8 @@ export default function VaultEditor(props: VaultEditorProps) {
       return;
     }
     let stopped = false;
+    let lastCanvasScan = 0;
     const detector = createTotpQrDetector();
-    if (!detector) {
-      setTotpQrStatus(t('txt_totp_qr_unsupported'));
-      return () => {
-        stopped = true;
-        stopTotpQrScanner();
-      };
-    }
     if (!navigator.mediaDevices?.getUserMedia) {
       setTotpQrStatus(t('txt_totp_qr_camera_unavailable'));
       return () => {
@@ -229,9 +246,26 @@ export default function VaultEditor(props: VaultEditorProps) {
         totpQrFrameRef.current = window.requestAnimationFrame(scan);
         return;
       }
+      try {
+        let value = '';
+        if (detector) {
           try {
             const results = await detector.detect(video);
-        const value = String(results[0]?.rawValue || '').trim();
+            value = String(results[0]?.rawValue || '').trim();
+          } catch {
+            // Fall back to jsQR when the native detector is present but not usable.
+          }
+        }
+        // The jsQR fallback runs a synchronous full-frame decode, so throttle
+        // it to a few times per second instead of every animation frame to
+        // avoid pegging the CPU while a code is being aligned.
+        if (!value) {
+          const now = performance.now();
+          if (now - lastCanvasScan >= 250) {
+            lastCanvasScan = now;
+            value = decodeTotpQrCanvas(video);
+          }
+        }
         if (value && applyTotpQrValue(value)) return;
       } catch {
         // Keep the camera active; transient frame decode failures are common.