refactor(nav): read navigation from context, drop the prop-drilling

VideoCard/PlayerModal read openChannel from useNavigationActions (the memo'd
VideoCard subscribes to the STABLE actions context, so a page/channel change
no longer re-renders it); NavSidebar/Header read page/setPage; ChannelPage
reads closeChannel; NotificationsPanel reads setPage. App sheds the matching
props, and Feed/VirtualFeed no longer thread onOpenChannel at all.

frontend/src/components/Feed.tsx

@@ -88,7 +88,6 @@ export default function Feed({
   ytSearch,
   onYtSearch,
   onExitYtSearch,
-  onOpenChannel,
   channelScoped,
 }: {
   filters: FeedFilters;
@@ -107,9 +106,6 @@ export default function Feed({
   ytSearch: string | null;
   onYtSearch: (q: string) => void;
   onExitYtSearch: () => void;
-  // 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;
@@ -308,13 +304,6 @@ export default function Feed({
     [qc]
   );
 
-  const handleOpenChannel = useCallback(
-    (channelId: string, channelName: string) => {
-      onOpenChannel?.(channelId, channelName);
-    },
-    [onOpenChannel]
-  );
-
   const onToggleSave = useCallback(
     (id: string, saved: boolean) => {
       setSavedOverrides((o) => ({ ...o, [id]: saved }));
@@ -447,7 +436,6 @@ export default function Feed({
               view={view}
               onState={onState}
               onToggleSave={onToggleSave}
-              onOpenChannel={handleOpenChannel}
               onResetState={onResetState}
               onOpen={openVideo}
               hasNextPage={false}
@@ -480,7 +468,6 @@ export default function Feed({
               queue={ytPlayerQueue}
               onClose={() => setActiveVideo(null)}
               onState={onState}
-              onOpenChannel={onOpenChannel}
             />
           </Suspense>
         )}
@@ -698,7 +685,6 @@ export default function Feed({
           view={view}
           onState={onState}
           onToggleSave={onToggleSave}
-          onOpenChannel={handleOpenChannel}
           onResetState={onResetState}
           onOpen={openVideo}
           hasNextPage={!!hasNextPage}
@@ -714,7 +700,6 @@ export default function Feed({
             queue={playerQueue}
             onClose={() => setActiveVideo(null)}
             onState={onState}
-            onOpenChannel={onOpenChannel}
           />
         </Suspense>
       )}