ShadowCat is a fully offline, single-file HTML page for moving data between two devices via QR codes — intended for old phones whose radios (BLE, NFC, etc.) are dead but whose cameras and browsers still work. Generate  — encode text into a single QR code. Scan  — decode a single QR via the camera. Send file  — pick a file, choose chunk size / FPS / ECC, hit Start. Cycles through  [header, chunk1…chunkN]  forever at the chosen FPS. Pause / Resume / Stop. Receive file  — start the camera and point at the sender. Header autodetects, progress bar fills in, missing-chunks grid shows which ones haven’t arrived yet. When complete, the file’s CRC is verified and a Download button appears. Practical notes for old phones A camera needs HTTPS or localhost — file:// won’t grant getUserMedia  permission. Serve with  python3 -m http.server 8000  and visit  http://<your-laptop-ip>:8000/qrcode.html  over the local network.…