fix(player): open the channel page from the player without it bouncing back

Two bugs made clicking the channel name in the video modal just close the player:
- the second PlayerModal mount (main feed path) was missing onOpenChannel, so the handler
  hit its no-op early return;
- opening the channel synchronously pushed the _chan history entry, which the player's own
  useBackToClose teardown (history.back on unmount) then immediately popped. Now the open is
  deferred to a one-shot popstate listener that fires AFTER that teardown, so the channel
  entry lands at the feed level. Verified: player → channel name → channel page; Back → feed
  (player does not reappear).

frontend/src/components/Feed.tsx

@@ -587,6 +587,7 @@ export default function Feed({
           startAt={activeVideo.startAt}
           onClose={() => setActiveVideo(null)}
           onState={onState}
+          onOpenChannel={onOpenChannel}
         />
       )}
       {isFetchingNextPage && (