[Feature] Add typed timeline tracks

[chdenat]

Problem

Timeline tracks are currently generic. The editor needs explicit track types so video, audio, text, graphics, marker, and custom tracks can expose the right icons, controls, and visual behavior.

Scope

Add a controlled type field to track definitions, with documented values including video and audio. Tracks may provide an optional icon and remain compatible with existing groups, clips, slots, selection, editing, drag and drop, and controlled updates.

The Add track control becomes a vertical typed button group:

• Add video track;
• Add audio track;
• Add text track;
• Add graphics track;
• Add custom track when configured.

In the compact toolbar, a plus button opens the same menu. Track type remains independent from visual group membership.

The complete model and UI design is documented in docs/track-types.md.

Media thumbnails, waveforms, linked audio/video editing, and Mediabunny playback are tracked separately in issue #8.

Acceptance criteria

• Tracks expose documented types with at least video and audio values.
• Track headers show accessible type icons and labels.
• The Add track action creates a selected track type through the vertical button group.
• The compact plus button opens the same typed menu.
• Existing controlled track behavior remains compatible.