Lower timeline z-index below sidebar so panel covers it when open

Sidebar uses z-index 10; set timeline to 9 so it stays visible on the
canvas but sits underneath the sidebar panel when expanded.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/snapshot_manager.js

@@ -894,7 +894,7 @@ const CSS = `
     display: flex;
     align-items: center;
     padding: 0 16px;
-    z-index: 1000;
+    z-index: 9;
     pointer-events: auto;
 }
 .snap-timeline-track {