feat(components): add playback speed control to audio player

[jefferypippitt]

## Changes
- Added `AudioPlayerSpeed` component with settings icon dropdown
- Added `AudioPlayerSpeedButtonGroup` component for button group variant
- Added `playbackRate` and `setPlaybackRate` to audio player context
- Integrated speed control into audio player demo
- Updated documentation with API reference
- Shows "Normal" for 1x speed, numerical values (0.5x, 1.5x, etc.) for others

## Demo
Audio player now has a settings icon (⚙️) on the right side of the progress bar that opens a dropdown with speed options from 0.25x to 2x.

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[louisjoecodes]

@jefferypippitt nice one!

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Project	Deployment	Preview	Comments	Updated (UTC)
ui	Ready	Preview	Comment	Oct 8, 2025 8:52am

[louisjoecodes]

Very nicely designed and implemented 🔥