[Feature] Group timeline tracks by group

[chdenat]

Problem

Timeline tracks are currently rendered as one flat list. Projects with several camera, audio, overlay, or auxiliary tracks need a way to organize related tracks visually.

Design

The proposed model adds controlled groups with stable ids, labels, optional icons, configurable colors, and collapsed state. Tracks receive an optional groupId. Grouped tracks remain individually selectable, editable, draggable, and addressable. Groups may be empty; a group can be removed only when it contains no tracks.

The UI includes:

• colored group headers spanning the legend and timeline;
• expanded groups with their individual track rows;
• collapsed groups with a compact clip overview row;
• drag and drop of a track onto an open or collapsed group header to assign it to that group;
• optional icons for groups and tracks;
• a vertical Add track / Add group button group;
• a compact plus button that opens the same button group when the toolbar is reduced.

The complete model and interaction design is documented in docs/issues/issue-6.md.

Acceptance criteria

• Group ordering and track membership are controlled by the host.
• Group colors and icons are represented in the public model and snapshots.
• A collapsed group remains a valid drag target.
• Removing a non-empty group is unavailable and never removes its tracks.
• Existing track, clip, slot, drag, selection, playback, and controlled update behavior remains compatible.
• README, specification, demo, and regression tests cover the implementation.