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/e2e/feed.spec.ts

@@ -47,4 +47,35 @@ test.describe("feed", () => {
       .poll(async () => (await renderedIds()).some((id) => id && !before.includes(id)))
       .toBe(true);
   });
+
+  // Bug 4 (Platform S4): the toolbar (show/content chips, count, sort, view) is fixed chrome — it
+  // pins in the shell's toolbar band above the scroller, so it stays put while the cards scroll.
+  test("the feed toolbar stays pinned while cards scroll", async ({ page }) => {
+    await openApp(page);
+    const count = page.getByTestId("feed-result-count");
+    await expect(count).toBeVisible();
+    const topBefore = await count.evaluate((el) => el.getBoundingClientRect().top);
+
+    await page.getByRole("main").evaluate((el) => el.scrollBy(0, 4000));
+
+    await expect(count).toBeInViewport();
+    const topAfter = await count.evaluate((el) => el.getBoundingClientRect().top);
+    expect(Math.abs(topAfter - topBefore)).toBeLessThan(4);
+  });
+
+  // Bug 3 (Platform S4): changing a filter is a fresh result set, so the scroll jumps back to the
+  // top rather than stranding you deep in the old list's offset.
+  test("a new filter resets the scroll to the top", async ({ page }) => {
+    await openApp(page);
+    await expect(page.getByTestId("video-card").first()).toBeVisible();
+
+    await page.getByRole("main").evaluate((el) => el.scrollBy(0, 4000));
+    expect(await page.getByRole("main").evaluate((el) => el.scrollTop)).toBeGreaterThan(500);
+
+    // Switch the show filter → a different result set → back to the top.
+    await page.getByTestId("feed-show-all").click();
+    await expect
+      .poll(() => page.getByRole("main").evaluate((el) => el.scrollTop))
+      .toBeLessThan(50);
+  });
 });