fix(datatable): the sticky header no longer shifts or fades on first scroll

The sticky <thead> jumped up ~8px and picked up the scroller's top fade
the moment you scrolled 1px — so it read as 'not fixed'. Two causes:
- the table sat under a pt-2 gap, so its natural position was 8px below
  the sticky top:0 it snapped to. Drop the top padding (the fixed band's
  controls row already gives the visual gap) → no shift.
- the page scroller's top edge fade turned on with the first scroll and
  masked the header's top. Add a fadeTop flag (PageShell → PageScroller →
  useScrollFade), off for the Channels + Audit pages whose scroll region
  starts with a sticky header. Feed/channel keep the top fade (it fades
  cards under the fixed toolbar, as intended).

Verified: header stays at a fixed offset with no fade change from scroll 0
through scrolled, on both the manager and the audit log. Suite 15/15.

frontend/src/components/PageScroller.tsx

@@ -63,11 +63,14 @@ function restoreScroll(el: HTMLElement, top: number): () => void {
 export default function PageScroller({
   children,
   scrollKey,
+  fadeTop = true,
 }: {
   children: ReactNode;
   scrollKey?: string;
+  // Off when the scroller's top is a sticky element (a table's sticky header) — see useScrollFade.
+  fadeTop?: boolean;
 }) {
-  const fade = useScrollFade();
+  const fade = useScrollFade(20, fadeTop);
   const fadeRef = fade.ref;
   const elRef = useRef<HTMLElement | null>(null);
   // Stable identity: an inline ref callback changes every render, which makes React detach and