feat(media): accessible AudioPlayer component

[bntvllnt]

Problem

The registry has no audio or media-player component, so any property with audio
content (podcasts, embedded clips) hand-rolls play/pause, scrubbing, duration,
and transcript UI — including the a11y that's easy to get wrong. Consumers import
components and override via props/className; there's nothing to import here.

Proposal

An accessible AudioPlayer: play/pause, seek/scrub, elapsed + duration
(tabular-nums), optional transcript toggle, optional playback-rate. Token-driven
colors, lucide-react icons, full keyboard support, aria-* on the scrubber,
respects prefers-reduced-motion.

Acceptance

• Keyboard: play/pause (Space/Enter), seek ±, Home/End
• Screen reader: labelled controls, aria-valuenow/aria-valuetext on the scrubber
• Token-driven styling (no literal colors)
• Works with a plain audio URL; transcript slot optional

[github-actions]

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