v0.1.1

Make calibration capture diagnose its own failures

Pressing Space silently no-op'd whenever latestYawRef was still null,
which gave the user no signal that anything was wrong. There are at
least three real causes (MediaPipe model still downloading, init
exception, no face yet detected) and none of them surfaced.

- Show init phase ("Loading face tracker…" / "Waiting for your face…"
  / "Tracker ready") with a live yaw readout, so the user can see when
  the pipeline is alive before pressing Space.
- Visible mini camera preview during calibration. Doubles as a sanity
  check that the webcam stream is up, and avoids `display: none` on
  the source <video>, which can suppress frame decoding in some
  WebView2 builds.
- "Capture this point" button as a backup for Space (and disabled
  until init is ready). Space presses on a not-yet-ready tracker now
  flash a transient hint instead of being eaten.
- Catch and surface init errors inline rather than swallowing them.

Bump to 0.1.1 so the next tag picks this commit up.