Add timeline track and clip creation controls

[Egoistian]

Summary

• add optional ClapTrack.category metadata and timeline store actions for creating tracks, setting empty-track categories, creating typed clips, and moving clips only to compatible tracks
• add left-track controls for + Track, category selection, and + Clip
• preserve explicit startTimeInMs when adding segments and keep track occupancy in sync when clips are assigned

Validation

• npx --yes bun@latest run --cwd packages/clap build
• npx --yes bun@latest run --cwd packages/timeline build
• git diff --check
• local browser QA harness: verified visible track controls, locked dropdown and store guard for occupied tracks, category change on an empty track, typed clip creation, and appending a new track

Notes

I could not complete packages/app build locally because its prepare step expects ../../node_modules/mediainfo.js/dist/MediaInfoModule.wasm, while the local Bun install layout placed that dependency under node_modules/.bun/.... The failure occurs before app compilation and appears unrelated to this timeline patch.

[MyTH-zyxeon]

Review assist for #10 / Boss bounty:

I checked this diff because it is one of the still-unreviewed timeline authoring attempts. The scope is useful and focused: it adds typed track creation, typed clip creation, track-category metadata, and compatible cross-track moves without introducing another broad app rewrite.

Acceptance points I would verify before choosing a canonical PR:

• Track identity: createTrack({ track: tracks.length }) can collide with sparse/deleted/reordered track arrays unless the app guarantees dense ids forever. A regression around delete/reorder/add would make this safer.
• Occupancy/category state: moving a segment updates occupied flags, but category changes are blocked only by occupied; tests should cover moving the last clip away, then changing the newly empty track type.
• Pointer math: drag-to-track uses canvas bounds plus scrollX/scrollY, leftBarTrackScaleWidth, and topBarTimeScaleHeight; a focused fixture for scroll/zoom/camera-offset behavior would reduce merge risk.
• Rejected moves: moveSegmentTo returns false on incompatible track type, but the UI path does not appear to surface why the drag was rejected. Maintainers may want a visible snap-back or disabled target state.
• UI integration: the absolute TrackControlsOverlay sits inside the timeline root at zIndex: 20; check it does not cover existing left-bar controls, time scale, or pointer interaction on narrow layouts.
• Build evidence: the package builds listed in the PR body are useful, but the app-level mediainfo.js prepare blocker should be separated from timeline acceptance so maintainers know exactly what remains unrelated.

This looks like a plausible timeline-native base or comparison point for #10, especially if the above cases are backed by store-level tests and one browser smoke path. It should be compared against #134/#136/#139/#140/#142/#145 before merging to avoid picking overlapping implementations.