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/Header.tsx

@@ -2,11 +2,11 @@ import { type ReactNode } from "react";
 import { useTranslation } from "react-i18next";
 import { Youtube } from "lucide-react";
 import type { FeedFilters, Me } from "../lib/api";
-import type { Page } from "../lib/urlState";
 import { moduleLabelKey, moduleOrder, stepModule } from "../lib/modules";
 import ModuleName from "./ModuleName";
 import SearchBar from "./SearchBar";
 import SyncStatus from "./SyncStatus";
+import { useNavigation, useNavigationActions } from "./NavigationProvider";
 
 // The floating top header: a row of glass pills over the content — a ModuleName pill (label +
 // accent dot + back/forward arrows), the module's SearchBar where it has one, and the per-user
@@ -21,8 +21,6 @@ export default function Header({
   setChannelsSearch,
   playlistsSearch,
   setPlaylistsSearch,
-  page,
-  setPage,
   onYtSearch,
   onGoToFullHistory,
 }: {
@@ -36,8 +34,6 @@ export default function Header({
   setChannelsSearch: (q: string) => void;
   playlistsSearch: string;
   setPlaylistsSearch: (q: string) => void;
-  page: Page;
-  setPage: (p: Page) => void;
   // Trigger a live YouTube search for the current term (feed only; hidden for the demo account,
   // which can't spend the shared quota).
   onYtSearch: (q: string) => void;
@@ -45,6 +41,8 @@ export default function Header({
   onGoToFullHistory: () => void;
 }) {
   const { t } = useTranslation();
+  const { page } = useNavigation();
+  const { setPage } = useNavigationActions();
   // The ◀/▶ arrows step cyclically through the user's available modules (derived from `me`, so
   // it stays correct as modules are added/gated). Labels of every active module feed ModuleName's
   // dynamic width so the pill is sized to the widest reachable title in the current language.