macro-clips slice 2: marker UI on the scrub ruler

- 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>

src/i18n/messages.ts

@@ -178,6 +178,11 @@ export const en = {
     showLocked: 'Show locked tracks',
     inPoint: 'In point — drag to set where playback and export start',
     outPoint: 'Out point — drag to set where playback and export end',
+    renameMarkerTitle: 'Rename marker',
+    renameMarkerMessage: 'Pick a unique name — used as a jump target in scripts.',
+    renameMarkerConfirm: 'Rename',
+    renameMarkerCollision: 'A marker named "{name}" already exists.',
+    renameMarkerRetry: 'Try again',
     stillFrameTooltip: 'Still frame {time} — holds until the next keyframe',
     keyframeTooltip: 'Keyframe {time} — drag to move, right-click for options',
     trimTooltip: '{name} · {duration}s{trims} · Drag edges to trim, double-click to open',