0.5.3: timeline name-col resize + playhead layering

- Track-name resize strip starts below the scrub bar and sits on the
  name-column side of the boundary, so scrubbing at time=0 is no
  longer intercepted by the resize hit area.
- Playhead (lane) and scrub handle get z-index 20 so they stay on top
  of the sticky track-name column (z-index 5) and the resize strip.
  The playhead stays fully visible at time=0 and when the lane is
  scrolled under the sticky column.

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

package.json

@@ -1,7 +1,7 @@
 {
   "name": "animatr",
   "private": true,
-  "version": "0.5.2",
+  "version": "0.5.3",
   "type": "module",
   "scripts": {
     "dev": "vite",