[Feature] Separate webcam track with visibility and position controls

[joaothaira]

Harvested from archived upstream: siddharthvaddem#584.

Request: webcam as its own timeline track with per-span visibility and position control.

Status in this fork: largely implemented. The segmented-webcam model already provides a webcam timeline row with multi-clip segments (add/remove/move spans = visibility control), plus corner/stack position presets and drag positioning. Remaining gap to evaluate before closing: per-segment position overrides (position is currently global, not per-clip).