fix: timeline filter dropdown z-index — add z-0 to scroll container

The scroll container's relative positioning created a stacking context
that rendered its children (z-30 sticky labels) above the toolbar's
dropdown (z-[60]). Adding z-0 to the scroll container explicitly places
it below the toolbar's z-20 stacking context, allowing dropdowns to
render on top.

Co-Authored-By: claude-flow <ruv@ruv.net>

apps/web/src/components/timeline/TimelineView.tsx

@@ -615,7 +615,7 @@ function TimelineViewContent({
       <div
         ref={scrollContainerRef}
         onScroll={handleContainerScroll}
-        className="app-surface relative flex-1 overflow-auto"
+        className="app-surface relative z-0 flex-1 overflow-auto"
       >
         {isInitialLoading ? (
           <div className="flex items-center justify-center py-24 text-sm text-gray-500 dark:text-gray-400">