FFmpeg is now automatically downloaded on first run if not found,
eliminating the need for vcpkg or manual FFmpeg installation on Windows.
Changes:
- Add FFmpeg availability check and auto-download on startup
- Use ffprobe_path() for auto-downloaded binaries in video decoder
- Track FFmpeg availability in app state with graceful degradation
- Show warning banner when video conversion is unavailable
- Skip video files when adding to queue if FFmpeg unavailable
- Simplify build instructions in README (remove vcpkg recommendation)
