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/NavSidebar.tsx

@@ -2,6 +2,7 @@ import { useEffect, useRef, useState, useSyncExternalStore } from "react";
 import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
 import { useQuery } from "@tanstack/react-query";
+import { useNavigation, useNavigationActions } from "./NavigationProvider";
 import {
   Activity,
   BarChart3,
@@ -40,8 +41,6 @@ import LanguageSwitcher from "./LanguageSwitcher";
 // a thin icon-only rail. Account actions sit at the bottom in a popover.
 export default function NavSidebar({
   me,
-  page,
-  setPage,
   onOpenAbout,
   onChangeLanguage,
   language,
@@ -49,8 +48,6 @@ export default function NavSidebar({
   onToggleCollapse,
 }: {
   me: Me;
-  page: Page;
-  setPage: (p: Page) => void;
   onOpenAbout: () => void;
   onChangeLanguage: (code: LangCode) => void;
   language: LangCode;
@@ -60,6 +57,8 @@ export default function NavSidebar({
   onToggleCollapse: () => void;
 }) {
   const { t } = useTranslation();
+  const { page } = useNavigation();
+  const { setPage } = useNavigationActions();
   // When unpinned, the rail sits slim (icons only) and peeks open into a labelled overlay on hover
   // or keyboard focus WITHOUT pushing content. Pinned keeps it expanded in-flow like before.
   // `collapsed` (persisted per-account) now means "unpinned".