feat(player): channel name opens the in-app channel page

The PlayerModal channel name now opens our channel page (closing the player first); a
small external-link icon beside it keeps the open-on-YouTube behaviour. Threaded
onOpenChannel from Feed into both PlayerModal mounts.

frontend/src/components/Feed.tsx

@@ -382,6 +382,7 @@ export default function Feed({
             startAt={activeVideo.startAt}
             onClose={() => setActiveVideo(null)}
             onState={onState}
+            onOpenChannel={onOpenChannel}
           />
         )}
       </div>