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
