fix: marker double-click blocked by scrub handle overlap

After a single click the playhead snapped to the marker's time, which
moved the scrub handle's hit area directly on top of the marker flag.
The second click then landed on the handle (z-index 20) instead of
the marker (z-index 18), so dblclick never fired. Raised marker
z-index to 21 so marker-hits win at overlapping positions — still
below modals/popups (900+).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/Timeline.vue

@@ -2885,7 +2885,9 @@ onBeforeUnmount(() => {
   position: absolute;
   top: 0;
   bottom: 0;
-  z-index: 18;
+  /* Above playhead (20) and brackets (19) so marker clicks/drags win
+     when the playhead has snapped onto the marker's position. */
+  z-index: 21;
   pointer-events: auto;
   cursor: ew-resize;
   touch-action: none;