feat: persistent audio mini-player docked above bottom nav

[barrydeen]

Summary

• Replaces the basic inline audio widget with a global mini-player that persists across feed scrolling and tab navigation, with system notification / lock-screen transport controls via MediaSessionService.
• Two-state dock: collapsed shows the author's avatar and display name with rewind-15 / play-pause / forward-15; swipe up to expand for a scrub slider, speed cycling (0.75→2.0x), and a close button. Swipe down to collapse.
• Single ExoPlayer owned by AudioPlayerController; the inline tap-to-play widget now reflects global playback state and hands off to the controller. Starting audio pauses any active PiP video so they don't fight.

Architecture

• AudioPlayerController — singleton with StateFlow<AudioPlaybackState> exposing play/togglePlayPause/seekTo/skipForward/skipBackward/cycleSpeed/close.
• AudioMediaSession — parallel to the existing VideoMediaSession; both register with the renamed WispPlaybackService (was VideoPlaybackService) which keeps the service alive while either session is active.
• FloatingAudioPlayer mounts in the root Scaffold's bottomBar slot above WispBottomBar so content naturally reserves space rather than being covered.
• RichContent gets an optional authorPubkey; PostCard passes event.pubkey so AudioTrack carries the kind-0 name + avatar into the dock.

Test plan

• Play an mp3 in the feed — inline player starts audio and the mini-player slides up above the bottom nav.
• Scroll the feed / switch tabs — player persists, audio uninterrupted, inline widget in the originating note mirrors playing state.
• Collapsed state shows author avatar + name + transport controls only (no slider, speed, or close).
• Swipe up to expand reveals slider / speed chip / close; swipe down collapses. Tap row also toggles.
• Scrub slider seeks; ±15 s buttons skip; speed chip cycles 0.75 → 1 → 1.25 → 1.5 → 2.
• Notification shade + lock-screen show title/artist/art with pause/resume.
• Starting a video while audio is playing pauses audio to avoid double-play.
• Close (×) kills the player and dismisses the notification.
• Swipe app from recents while paused → service stops. While playing → notification persists until closed.