fix(shell): scroll restore on Back + reset on new filter (bugs 1, 3)

The shell owns the scroller, so it owns the scroll policy. PageScroller
now takes a scrollKey identifying the logical view and keeps a per-view
offset in a module-level map, recorded live on scroll:
- same key returning (feed -> channel -> Back, filters unchanged) restores
  where you were (bug 1) -- the feed's scroller unmounts, but the saved
  offset survives and is re-applied, retried over a few frames so the
  virtualized list has time to reach its full height;
- a new key (feed filters changed) starts at the top of the fresh result
  set (bug 3). q is normalised out of the feed key so typing doesn't reset.

Recording on scroll rather than at unmount matters: by cleanup time the
scroller's scrollTop is already zeroed, so a cleanup-read saved 0.

Also fixes a Maximum-update-depth loop this introduced: the combined
element/fade ref was an inline callback, so React re-attached it every
commit and re-ran useScrollFade's state setter. Bound it with useCallback.

Adds E2E locks for bugs 1, 3 and 4 (restore on Back, reset on filter,
toolbar stays pinned). Full suite 14/14 green on :5173; tsc + knip clean.

frontend/src/components/PageShell.tsx

@@ -38,10 +38,13 @@ export default function PageShell({
   header,
   banners,
   children,
+  scrollKey,
 }: {
   header?: ReactNode;
   banners?: ReactNode;
   children: ReactNode;
+  // Identifies the logical view for the scroll restore/reset policy — see PageScroller.
+  scrollKey?: string;
 }) {
   const [toolbarSlot, setToolbarSlot] = useState<HTMLDivElement | null>(null);
   return (
@@ -50,7 +53,7 @@ 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>{children}</PageScroller>
+        <PageScroller scrollKey={scrollKey}>{children}</PageScroller>
       </div>
     </ToolbarSlot.Provider>
   );