fix(channel): no-banner fallback bar matches the banner height (avatar/Back overlap)

The no-banner placeholder was a short h-14 (56px) bar while a real banner renders
~150px, so the -mt-8 avatar rode up into the absolutely-positioned Back button on
banner-less channels (e.g. Billet Box). Give the fallback the same aspect-ratio +
maxHeight as the banner container so the overlapping avatar has the same room and
the header layout is consistent whether or not a channel has a banner.

frontend/src/components/ChannelPage.tsx

@@ -228,7 +228,13 @@ export default function ChannelPage({
             />
           </div>
         ) : (
-          <div className="h-14 w-full rounded-2xl bg-surface" />
+          // No banner (or a dead one): a neutral bar the SAME height as a real banner, so the
+          // overlapping avatar + the Back button have the same room and don't collide (a short bar
+          // let the -mt-8 avatar ride up into the Back button).
+          <div
+            className="w-full rounded-2xl bg-surface"
+            style={{ aspectRatio: "2560 / 423", maxHeight: "150px" }}
+          />
         )}
         <button
           onClick={onBack}