feat(shell): channel chrome is fully fixed, only the grid scrolls

Reworks the channel detail per UAT: instead of the banner scrolling away
and a sticky sub-header (which felt odd to scroll and needed an opaque bar
that clashed with the design even at rest), the ENTIRE chrome — banner,
identity, tabs, toolbar — is fixed and only the video grid scrolls.

Mechanism: ChannelPage wraps its whole chrome in <PageToolbar> so it
portals into the shell's fixed band (like the feed toolbar); the
channelScoped Feed portals its toolbar into a slot at the bottom of that
chrome. No sticky, no opaque bar — the chrome is transparent over the page
bg because nothing scrolls under it (the band sits above the scroller).

E2E updated: the channel chrome (title + toolbar) stays at a fixed
viewport position while the grid scrolls. Suite 15/15, tsc clean.

frontend/e2e/channel.spec.ts

@@ -56,15 +56,17 @@ test.describe("channel view", () => {
       .toBeGreaterThan(saved - 500);
   });
 
-  // Platform S4b: the channel page's banner/identity scroll away, but the tabs + feed toolbar stick
-  // to the top (a sticky sub-header) so only the video grid scrolls.
-  test("tabs + toolbar stay pinned while the channel videos scroll", async ({ page }) => {
+  // Platform S4b: the whole channel chrome (banner, identity, tabs, toolbar) is fixed — it stays
+  // put while only the video grid scrolls.
+  test("the channel chrome stays fixed while the video grid scrolls", async ({ page }) => {
     await openApp(page);
     await page.getByTestId("video-card-channel").first().click();
     const count = page.getByTestId("feed-result-count");
+    const title = page.getByTestId("channel-title");
     await expect(count).toBeVisible();
     await expect(page.getByTestId("video-card").first()).toBeVisible();
-    await expect(page.getByTestId("channel-title")).toBeInViewport();
+    const titleTop = await title.evaluate((el) => el.getBoundingClientRect().top);
+    const countTop = await count.evaluate((el) => el.getBoundingClientRect().top);
 
     // Keep nudging until the grid is tall enough to actually scroll (it may still be filling in).
     await expect
@@ -74,9 +76,8 @@ test.describe("channel view", () => {
       })
       .toBeGreaterThan(400);
 
-    // Banner/identity scrolled away, but the tabs+toolbar stuck near the top.
-    await expect(page.getByTestId("channel-title")).not.toBeInViewport();
-    await expect(count).toBeInViewport();
-    expect(await count.evaluate((el) => el.getBoundingClientRect().top)).toBeLessThan(130);
+    // The chrome didn't move — banner title and toolbar are in exactly the same place.
+    expect(await title.evaluate((el) => el.getBoundingClientRect().top)).toBe(titleTop);
+    expect(await count.evaluate((el) => el.getBoundingClientRect().top)).toBe(countTop);
   });
 });

frontend/src/components/ChannelPage.tsx

@@ -5,7 +5,7 @@ import { useNavigationActions } from "./NavigationProvider";
 import { ArrowLeft, Ban, Check, ExternalLink, Loader2, Plus, RefreshCw, SlidersHorizontal } from "lucide-react";
 import Avatar from "./Avatar";
 import Feed from "./Feed";
-import { PageToolbarSlot } from "./PageShell";
+import { PageToolbar, PageToolbarSlot } from "./PageShell";
 import { useConfirm } from "./ConfirmProvider";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
@@ -77,9 +77,10 @@ export default function ChannelPage({
   const qc = useQueryClient();
   const confirm = useConfirm();
   const [tab, setTab] = useState<"videos" | "about">("videos");
-  // The sticky sub-header hosts the tabs AND the channel feed's toolbar: the Feed portals its
-  // toolbar into this node (via the overridden PageToolbarSlot below) so both ride together and
-  // stay put while the banner/identity scroll away above them.
+  // The whole channel chrome (banner, identity, tabs, toolbar) is fixed: it portals into the shell's
+  // fixed band so only the video grid scrolls. This node sits at the bottom of that chrome and is
+  // where the channelScoped Feed portals ITS toolbar (via the overridden PageToolbarSlot below), so
+  // the toolbar lands under the tabs inside the fixed band rather than in the scroller.
   const [chromeSlot, setChromeSlot] = useState<HTMLDivElement | null>(null);
   // The banner URL is valid, but googleusercontent intermittently throttles it when the channel
   // page fires the banner + avatar + ~16 thumbnails in one burst — the request is dropped and the
@@ -210,10 +211,12 @@ export default function ChannelPage({
       ? "pb-2 text-sm font-medium border-b-2 border-fg text-fg"
       : "pb-2 text-sm text-muted hover:text-fg border-b-2 border-transparent";
 
-  // The channel page owns no scroller of its own: it's a PageShell config (no shared header, no
-  // rail, its banner scrolls from the top) so there's ONE page scroller in the app, not two.
+  // The channel page owns no scroller of its own (PageShell provides the one app scroller). Its
+  // whole chrome is fixed — wrapped in <PageToolbar> it portals into the shell's fixed band, so the
+  // banner/identity/tabs/toolbar stay put and only the video grid scrolls.
   return (
     <>
+      <PageToolbar>
       {/* Banner + back — OPTION C: inset, rounded card (not full-bleed) */}
       <div className="relative px-4 sm:px-6 pt-3">
         {ch?.banner_url && bannerAttempt <= MAX_BANNER_RETRIES ? (
@@ -299,16 +302,11 @@ export default function ChannelPage({
             {t("channel.loadingVideos")}
           </div>
         )}
-      </div>
 
-      {/* Sticky sub-header: the tabs and (on the Videos tab) the feed toolbar ride here and stay put
-          while the banner + identity scroll away above them. The toolbar portals into `chromeSlot`
-          below via the overridden PageToolbarSlot; only the video grid scrolls. */}
-      <div className="sticky top-0 z-20 bg-surface border-b border-border">
         {/* Tabs — the channel actions ride along here rather than off in the header's top-right
             corner: there are only ever two tabs, so there's room, and it keeps the things you click
             within a few pixels of each other instead of across the page. */}
-        <div className="px-4 sm:px-6 pt-2 flex items-center gap-4">
+        <div className="flex items-center gap-4 mt-3 border-b border-border">
           <button onClick={() => setTab("videos")} className={tabClass(tab === "videos")}>
             {t("channel.tabVideos")}
           </button>
@@ -375,10 +373,14 @@ export default function ChannelPage({
               ))}
           </div>
         </div>
-        <div ref={setChromeSlot} />
       </div>
+      {/* The channelScoped Feed portals its toolbar into this node — the bottom of the fixed chrome,
+          just under the tabs. */}
+      <div ref={setChromeSlot} />
+      </PageToolbar>
 
-      {/* Tab content — the feed's toolbar portals up into the sticky sub-header via this override. */}
+      {/* Tab content — the feed's toolbar portals up into the fixed chrome via this slot override;
+          only the grid (and About) render here, in the scroller. */}
       <PageToolbarSlot.Provider value={chromeSlot}>
       {tab === "videos" ? (
         <>

frontend/src/components/Feed.tsx

@@ -655,7 +655,7 @@ export default function Feed({
   );
 
   return (
-    <div className={channelScoped ? "px-4 sm:px-6 pb-4" : "px-4 pb-4 pt-2"}>
+    <div className={channelScoped ? "px-4 sm:px-6 pb-4 pt-3" : "px-4 pb-4 pt-2"}>
       {/* The toolbar (+ active-filter chips) is FIXED chrome: it portals into a PageToolbarSlot so
           it stays put while the cards scroll (bug 4). On the feed page that slot is the shell's top
           band; on a channel page ChannelPage overrides the slot with its own sticky sub-header, so