fix(channel): drop channel-constant sort options on the channel page

Subscribers and Channel-priority sorts are meaningless when the feed is scoped to one channel
(both are constant across its videos), so hide them there via a channelScoped flag on Feed.
The main feed keeps all sorts.

frontend/src/components/ChannelPage.tsx

@@ -255,6 +255,7 @@ export default function ChannelPage({
             onYtSearch={() => {}}
             onExitYtSearch={() => {}}
             onOpenChannel={onOpenChannel}
+            channelScoped
           />
           {!ch?.subscribed && nextToken && (
             <div className="flex justify-center pb-6">