Add timeline track and clip creation controls

[elianguitarra]

Summary

• Adds timeline store actions for creating typed tracks and clips.
• Adds a small timeline overlay with a track-type selector, target-track selector, and buttons to add tracks/clips.
• Creates new clips at the current timeline cursor with valid Clap segment defaults.
• Enables clip dragging on the timeline with step snapping, zero-time clamping, collision rejection, and compatible cross-track moves.

Details

This is a focused first pass for issue #10. It enables dynamic track creation and dynamic clip creation from the editor UI:

• New tracks can be created with a selected Clap segment category.
• New clips can be created on a new track or an existing visible track.
• Clips created on an existing track use the track category as the source of truth.
• Timeline metrics are recalculated after track/clip creation and clip drag moves.
• Dragging a clip horizontally preserves its duration and clamps it at time zero.
• Dragging a clip onto another track is accepted only when that track has the same category.
• Drag moves are rejected when they would collide with another clip on the target track.

Verification

• git diff --check passes.
• Local Bun runtime found at .tools/bun/bin/bun.exe.
• bun build packages/timeline/src/index.tsx --outfile=packages/timeline/dist/index.js ... passes.
• Timeline declaration emission passes when invoking TypeScript directly through bundled Node:
  node ./node_modules/typescript/lib/tsc.js --emitDeclarationOnly --project packages/timeline/tsconfig.types.json
• bun build packages/app/src/components/core/timeline/index.tsx --outfile=.tools/timeline-component-check.js ... passes.

Patch

Public patch link: https://paste.rs/dMU4V

Related issue

Closes #10