Add timeline quick actions for creating typed tracks and clips

[ink00Yuan]

Summary

This PR adds a first pass at dynamic timeline editing:

• Add typed tracks from the timeline UI.
• Create clips at the current timeline cursor.
• Prefer same-category tracks when creating clips.
• Move clips horizontally on the timeline.
• Allow moving clips to empty or same-category tracks.
• Reject moves to tracks with incompatible categories.

Implementation Notes

• Added addTrack, createClip, and moveSegment to the timeline store.
• Added TimelineQuickActions with a category selector and quick Add track/Add clip buttons.
• Updated existing segment pointer handling so hover does not automatically put a segment into dragging mode.
• During drag, the segment center follows the pointer and the store rejects incompatible target tracks.
• Empty tracks adopt the moved/created segment category when they become occupied.
• Empty audio clips without an assetUrl no longer try to load an audio buffer.

Validation

git diff --check
git diff --check main..HEAD
npm exec --yes bun@1.0.25 -- run build:clap
npm exec --yes bun@1.0.25 -- run build:timeline