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/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