fix: timeline filter dropdown rendered behind scroll container

Add relative z-20 to TimelineToolbar so ProjectCombobox and
ResourceCombobox dropdowns (z-[60]) render above the scroll
container below. The scroll container's relative positioning
was creating a stacking context that overlapped the dropdowns.

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

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

@@ -90,7 +90,7 @@ export function TimelineToolbar({
   }
 
   return (
-    <div className="app-toolbar flex flex-wrap items-center justify-between gap-3">
+    <div className="app-toolbar relative z-20 flex flex-wrap items-center justify-between gap-3">
       <div className="flex items-center gap-3">
         <ProjectCombobox
           value={filters.projectIds[0] ?? null}