Scan board host

Local board-scan P0: capture → 4 corners → stub FEN. Catalog demo still skips the engine.

Dummy flow for the photo-to-FEN overlay. No camera permission and no DeepSeek. Open the dialog, then use the in-overlay demo steps (or the buttons below while it is closed). Capture is a sample board photo; Dummy scan plays reading → result; Warnings and Quota used are the other states.