fix(web): pin the tasks table header while scrolling

sticky top-0 on each <th> (not the <thead> itself — more consistent
sticky support across browsers for table headers) plus a background so
scrolled rows don't show through underneath it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/pages/Overview.svelte

@@ -87,7 +87,7 @@
     {:else}
       <table class="w-full text-sm">
         <thead>
-          <tr class="border-b text-left text-xs text-muted-foreground">
+          <tr class="border-b text-left text-xs text-muted-foreground [&>th]:sticky [&>th]:top-0 [&>th]:z-10 [&>th]:bg-card/95 [&>th]:backdrop-blur">
             <th class="w-36 px-4 py-2 font-medium">Status</th>
             <th class="px-4 py-2 font-medium">Task</th>
             <th class="hidden px-4 py-2 font-medium md:table-cell">Summary</th>