feat: Fast-forward/rewind within a track

[qutad]

Summary

• Preserve normal previous/next behavior for short taps.
• Fast-forward at 2x while holding the next button.
• Play decoded audio backward at 2x while holding the previous button.
• Restore the original playback state when the button is released.
• Keep scanning within the current track.
• Preserve ReplayGain, audio focus, audio effects, and media-session progression.
• Add tests for PCM frame reversal and amplification.

Implementation

Forward scanning uses ExoPlayer's playback-speed support.

Reverse scanning uses a bounded MediaExtractor/MediaCodec pipeline that decodes PCM windows, reverses complete interleaved frames, and streams them through AudioTrack.

Testing

• ./gradlew spotlessCheck
• ./gradlew :app:testDebugUnitTest
• ./gradlew :app:assembleDebug
• Manual testing on: Google Pixel 9a

Limitations

Reverse playback depends on Android's platform MediaCodec support. A format that Auxio can play only through its FFmpeg renderer may not support reverse scanning.

Closes #1413