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/NotificationsPanel.tsx

@@ -20,8 +20,8 @@ import {
   type VideoWatchedMeta,
 } from "../lib/notifications";
 import { channelYouTubeUrl, relativeFromMs, relativeTime } from "../lib/format";
-import type { Page } from "../lib/urlState";
 import { LEVEL_STYLE } from "./Toaster";
+import { useNavigationActions } from "./NavigationProvider";
 
 // The single notification center. "System" = durable, server-backed notifications (inbox
 // phase 1). "Activity" = the client-side transient events (action confirmations, errors, with
@@ -32,15 +32,14 @@ const POLL_MS = 8000;
 export default function NotificationsPanel({
   filters,
   setFilters,
-  setPage,
   onFocusChannel,
 }: {
   filters: FeedFilters;
   setFilters: (f: FeedFilters) => void;
-  setPage: (p: Page) => void;
   onFocusChannel: (name: string) => void;
 }) {
   const { t } = useTranslation();
+  const { setPage } = useNavigationActions();
   const qc = useQueryClient();
   const q = useLiveQuery<{ items: AppNotification[]; total: number }>(
     ["notifications"],