A cancel that landed while the finished .mov was being written still
renamed it over the chosen file. The rename is now the only step that
touches the destination, and a cancel before it removes the partial
file and reports the cancel. A test cancels mid-write over an existing
video and finds it intact; it fails without the check.

While Transparent background is ticked, the quality cards get no tap
at all, so they take no keyboard focus either, not just no clicks.

Tests now pin every keyframe (every 60 frames and the last), cover a
row that opens with a skip past 254 pixels, and check the video
preference survives a restart.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>