Updated the convert menu to default to source aspect ratio instead of 16:9,
which better preserves the original video's aspect ratio by default.
Changes:
- Initial state default: 16:9 → Source
- Empty fallback default: 16:9 → Source
- Reset button default: 16:9 → Source
- Clear video default: 16:9 → Source
- Updated hint label to reflect new default
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
