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/App.tsx

@@ -632,6 +632,17 @@ export default function App() {
     <div className="flex-1 grid place-items-center text-muted">{t("common.loading")}</div>
   );
 
+  // Scroll restore/reset key for the shared normal-page scroller (see PageScroller): the same
+  // page/feed-filters returns you to where you were (bug 1); a changed feed filter is a new key, so
+  // the fresh result set starts at the top (bug 3). `q` is normalised out so typing in the search
+  // box doesn't reset the scroll on every keystroke; the live-YouTube sub-view gets its own key.
+  const normalScrollKey =
+    page !== "feed"
+      ? `page:${page}`
+      : ytSearch != null
+        ? `yt:${ytSearch}`
+        : `feed:${JSON.stringify({ ...filters, q: "" })}`;
+
   return (
     <div className="h-screen flex text-fg">
       <NavSidebar
@@ -689,7 +700,7 @@ export default function App() {
       )}
       <div className="relative flex-1 min-w-0 flex flex-col">
         {channelView ? (
-          <PageShell>
+          <PageShell scrollKey={`channel:${channelView.id}`}>
           <Suspense fallback={pageFallback}>
             <ChannelPage
               key={channelView.id}
@@ -718,6 +729,7 @@ export default function App() {
           </PageShell>
         ) : (
           <PageShell
+            scrollKey={normalScrollKey}
             header={
               <Header
                 me={meQuery.data!}