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/VirtualFeed.tsx

@@ -58,7 +58,6 @@ export interface VirtualFeedProps {
   view: FeedView;
   onState: (id: string, status: string) => void;
   onToggleSave: (id: string, saved: boolean) => void;
-  onOpenChannel: (channelId: string, channelName: string) => void;
   onResetState: (id: string) => void;
   onOpen: (v: Video, startAt?: number | null) => void;
   hasNextPage: boolean;
@@ -71,7 +70,6 @@ export default function VirtualFeed({
   view,
   onState,
   onToggleSave,
-  onOpenChannel,
   onResetState,
   onOpen,
   hasNextPage,
@@ -187,7 +185,6 @@ export default function VirtualFeed({
                   width={FEED_VIEW_SPEC[view].thumb ? 0 : itemWidth}
                   onState={onState}
                   onToggleSave={onToggleSave}
-                  onOpenChannel={onOpenChannel}
                   onResetState={onResetState}
                   onOpen={onOpen}
                 />