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/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 ? (