[Feature] Add audio/video media support with Mediabunny

[chdenat]

Problem

Media tracks need source-aware visualizations, synchronized video/audio editing, and playback through Mediabunny.

Scope

Add a media model for typed video and audio tracks that supports:

• Mediabunny-generated video thumbnails;
• Mediabunny-generated audio waveform peaks;
• separate source and timeline positions;
• explicit links between compatible video and audio tracks;
• synchronized move, trim, split, and delete behavior;
• shared playback-rate calculation when linked clip duration changes;
• Mediabunny playback for video-only, audio-only, and linked audio/video clips;
• host-controlled synchronization through currentTimeMillis.

The complete model, playback flow, and UI design is documented in docs/audio-video-mediabunny.md.

Issue #7 covers the base typed track model and typed track creation controls.

Acceptance criteria

• Video clips display generated thumbnails when analysis is available.
• Audio clips display generated waveform peaks when analysis is available.
• Compatible tracks can be linked and unlinked explicitly.
• Linked edits preserve synchronization and expose the resulting playback rate.
• Mediabunny decodes and plays active video, audio, and linked audio/video clips.
• Analysis, decoder, audio, frame, and object URL resources are released correctly.
• Existing typed tracks and controlled timeline behavior remain compatible.