Add skip-back and playback speed controls to speakers audio player
- Add CSS for .spk-player-controls, .spk-skip-back, and .spk-speed-toggle.
- Wrap the audio element in a flex control bar with a 5s skip-back button and speed toggle button.
- Wire skip-back to clamp currentTime with Math.max(0, currentTime - 5).
- Wire speed toggle to cycle playbackRate through [1, 1.5, 2] and update button label.
- Keep no-audio-file behavior and sentence play button handling unchanged.