polish(channel): tighten toolbar-to-grid gap + restore top scroll fade

Two UAT nits on the channel page:
- the chip-row-to-cards gap was ~36px (toolbar pb-3 + wrapper pb-2 + grid
  pt-3 stacking); drop the wrapper/grid pads so the toolbar's own pb-3 is
  the only gap (~15px).
- restore the page scroller's TOP edge fade. It was dropped when the chrome
  was a sticky sub-header (the fade bit into it); now the chrome is a fixed
  band ABOVE the scroller, so the top fade correctly hints at cards
  scrolling up under the toolbar. Reverts the useScrollFade edges option.

Suite 15/15, tsc + knip clean.

frontend/src/components/PageScroller.tsx

@@ -67,9 +67,7 @@ export default function PageScroller({
   children: ReactNode;
   scrollKey?: string;
 }) {
-  // Only the bottom edge fades: the top is bounded by the shell's fixed chrome (header / toolbar
-  // band) or a module's sticky sub-header, so a top fade would just eat into that chrome.
-  const fade = useScrollFade(20, { top: false });
+  const fade = useScrollFade();
   const fadeRef = fade.ref;
   const elRef = useRef<HTMLElement | null>(null);
   // Stable identity: an inline ref callback changes every render, which makes React detach and