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/App.tsx

@@ -734,6 +734,9 @@ export default function App() {
         ) : (
           <PageShell
             scrollKey={normalScrollKey}
+            // Channels + Audit render a table with a sticky header at the scroller top; the top
+            // fade would sit on it and flip on with the first scroll, so turn it off there.
+            fadeTop={page !== "channels" && page !== "audit"}
             header={
               <Header
                 me={meQuery.data!}

frontend/src/components/Channels.tsx

@@ -577,7 +577,10 @@ export default function Channels({
         )}
       </div>
     ) : (
-      <div className="px-4 pb-4 pt-2 max-w-7xl mx-auto">
+      /* No top padding: the sticky header sits flush at the scroller top so it doesn't shift when
+         `position: sticky` engages on the first scroll (the band's controls row above gives the
+         visual gap). */
+      <div className="px-4 pb-4 max-w-7xl mx-auto">
         {/* Channel table — only the rows scroll; the header row is sticky, and the controls row
             (status chips + pager) rides in the fixed band right above it (controlsInBand). */}
         {channelsQuery.isLoading ? (

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

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>
   );

frontend/src/lib/useScrollFade.ts

@@ -7,7 +7,10 @@ import { useEffect, useState, type CSSProperties } from "react";
 // to attach to the scroller and the mask `style` to spread onto it. Uses a node-state callback
 // ref (not useRef) so the effect (re)runs whenever the element attaches — e.g. a SidePanel that
 // mounts collapsed and only renders its scroll body once expanded still wires up correctly.
-export function useScrollFade(fadePx = 20): {
+// `fadeTop` gates the top edge only. Pass false when the scroller's top is a sticky element (a
+// table's sticky header): fading there would eat into the header instead of hinting at content, and
+// the header would visibly change as the fade turns on with the first scroll.
+export function useScrollFade(fadePx = 20, fadeTop = true): {
   ref: (node: HTMLElement | null) => void;
   style: CSSProperties;
 } {
@@ -53,7 +56,7 @@ export function useScrollFade(fadePx = 20): {
     };
   }, [el]);
 
-  const mask = `linear-gradient(to bottom, transparent, #000 ${fade.up ? fadePx : 0}px, #000 calc(100% - ${
+  const mask = `linear-gradient(to bottom, transparent, #000 ${fade.up && fadeTop ? fadePx : 0}px, #000 calc(100% - ${
     fade.down ? fadePx : 0
   }px), transparent)`;
   return { ref: setEl, style: { maskImage: mask, WebkitMaskImage: mask } };