fix(playlists): stop the table header shifting up on first scroll (E9 12B)

The playlist content wrapper carried `pt-3`, so its sticky column header sat
12px below the scroller top at rest and jumped up to `top:0` on the first
scroll. Drop the top padding so the header is flush from rest and never
shifts — the fixed toolbar band above supplies the breathing room, matching
the channel table's already-correct pattern.

frontend/src/components/Playlists.tsx

@@ -599,7 +599,10 @@ export default function Playlists() {
   }
 
   return (
-    <div className="px-4 pb-4 pt-3">
+    // No top padding: the sticky column header must sit flush at the scroller top so it doesn't
+    // shift up when `position: sticky` engages on the first scroll (the fixed toolbar band above
+    // gives the breathing room). Same reason as the channel table — see Channels.tsx.
+    <div className="px-4 pb-4">
       {/* Header (title, actions, view controls) is fixed chrome — it portals into the shell's
           fixed band, so only the video list scrolls. */}
       <PageToolbar>