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/PageShell.tsx

@@ -41,12 +41,15 @@ export default function PageShell({
   banners,
   children,
   scrollKey,
+  fadeTop = true,
 }: {
   header?: ReactNode;
   banners?: ReactNode;
   children: ReactNode;
   // Identifies the logical view for the scroll restore/reset policy — see PageScroller.
   scrollKey?: string;
+  // Off when this page's scroll region starts with a sticky header — see PageScroller.
+  fadeTop?: boolean;
 }) {
   const [toolbarSlot, setToolbarSlot] = useState<HTMLDivElement | null>(null);
   return (
@@ -55,7 +58,9 @@ export default function PageShell({
       <div className={`flex-1 min-w-0 min-h-0 flex flex-col ${header ? "pt-[var(--hdr-h)]" : ""}`}>
         {banners}
         <div ref={setToolbarSlot} className="shrink-0" />
-        <PageScroller scrollKey={scrollKey}>{children}</PageScroller>
+        <PageScroller scrollKey={scrollKey} fadeTop={fadeTop}>
+          {children}
+        </PageScroller>
       </div>
     </PageToolbarSlot.Provider>
   );