v1.7

New feature: Create Radio Video (File → Create Radio Video…)
Combine a static cover image + an audio file into a video — for radio /
podcast program videos where the audio matters and the video is just a
static cover. The first (and only) re-encoding path in the app.

- Audio-only libmpv preview player with Mark In/Out to optionally trim
- Resolution presets: Native / 1080×1080 / 1920×1080 / 1280×720
- Progress bar + cancel

Export robustness fixes:
- Dedupe finished() (was emitting twice on crash/cancel)
- Cancel button now actually stops FFmpeg
- Fix QProgressDialog leak
- moveUp/moveDown no longer deselect at list boundaries
- Fix mono ogg/opus export (vorbis/opus reject -b:a 320k on mono)
- Trim ffmpeg banner from error messages

Cleanup: remove tracked stray test file; gitignore media.