fix: decode uploaded TOTP QR images when BarcodeDetector is unavailable

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",