fix(design): the table header takes the B9 fade utility, and every size is dvh, not vh (E28 S3d4)

The table header carried an arbitrary-value copy of fade-below; a contract refuses a hand gradient of a surface colour. 100vh is the viewport with a tablet browser's toolbar retracted, so h-screen / min-h-screen / static vh hid the last rows under it; a contract holds dvh (the Plex player stage changes with S3f, where it is played on the tablet).

frontend/src/components/ui/tableHeader.tsx

@@ -211,7 +211,7 @@ export function ColumnHead<T>({
   const [openFilter, setOpenFilter] = useState<string | null>(null);
 
   return (
-    <thead className="sticky top-0 z-base after:pointer-events-none after:absolute after:inset-x-0 after:top-full after:h-4 after:bg-[linear-gradient(to_bottom,var(--bg),transparent)] after:content-['']">
+    <thead className="sticky top-0 z-base fade-below">
       <tr className="text-muted [&>th]:shadow-[inset_0_-1px_0_var(--border)]">
         {columns.map((col) => {
           const isSorted = sort?.key === col.key;