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.