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">

frontend/src/components/Feed.tsx

@@ -78,6 +78,7 @@ export default function Feed({
   onYtSearch,
   onExitYtSearch,
   onOpenChannel,
+  channelScoped,
 }: {
   filters: FeedFilters;
   setFilters: (f: FeedFilters) => void;
@@ -95,8 +96,14 @@ export default function Feed({
   // Open a channel's dedicated page (from a card's channel name / avatar). Threaded down to
   // the cards via VirtualFeed.
   onOpenChannel?: (channelId: string, channelName?: string) => void;
+  // True when this feed is scoped to a single channel (the channel page). Drops sort options
+  // that are constant within one channel (subscribers, priority) — they'd be meaningless.
+  channelScoped?: boolean;
 }) {
   const { t } = useTranslation();
+  const sortKeys = channelScoped
+    ? SORT_KEYS.filter((k) => k !== "subscribers" && k !== "priority")
+    : SORT_KEYS;
   const [overrides, setOverrides] = useState<Record<string, string>>({});
   const [savedOverrides, setSavedOverrides] = useState<Record<string, boolean>>({});
   // The open player: which video and where to start (null = resume from saved position).
@@ -519,7 +526,7 @@ export default function Feed({
           className="bg-card border border-border rounded-lg px-2 py-1.5 text-sm outline-none focus:border-accent"
         >
           {/* Relevance only ranks meaningfully when there's a search term — offer it then. */}
-          {(filters.q.trim() ? (["relevance", ...SORT_KEYS] as SortKey[]) : SORT_KEYS).map((k) => (
+          {(filters.q.trim() ? (["relevance", ...sortKeys] as SortKey[]) : sortKeys).map((k) => (
             <option key={k} value={k}>
               {t("feed.sortKey." + k)}
             </option>