- marker flags render on the scrub ruler with color, label, and a dimmed
vertical guideline; z-index sits below brackets and playhead
- click marker to jump the playhead, drag to retime (snaps), double-click
to rename via prompt dialog, 'm' adds a marker at the playhead and
opens the rename prompt
- rename collision shows a dialog and loops back into the prompt
- selectedMarkerId tracks the active marker; Delete/Backspace removes it,
Escape clears the selection
- document keydown now gates m/Delete/Backspace on editable targets but
keeps Escape universal
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>