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/ChannelPage.tsx

@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import { useNavigationActions } from "./NavigationProvider";
 import { ArrowLeft, Ban, Check, ExternalLink, Loader2, Plus, RefreshCw, SlidersHorizontal } from "lucide-react";
 import Avatar from "./Avatar";
 import Feed from "./Feed";
@@ -57,8 +58,6 @@ export default function ChannelPage({
   me,
   view,
   setView,
-  onBack,
-  onOpenChannel,
   onShowInFeed,
 }: {
   channelId: string;
@@ -68,14 +67,13 @@ export default function ChannelPage({
   // toolbar carries the same switcher and a change here is the same account-wide choice.
   view: FeedView;
   setView: (v: FeedView) => void;
-  onBack: () => void;
   // Narrow the real feed to this channel and go there. Offered only for a channel you're
   // subscribed to: the feed defaults to scope "my", so filtering it to a channel you don't
   // follow would just land you on an empty page.
   onShowInFeed: () => void;
-  onOpenChannel: (id: string, name?: string) => void;
 }) {
   const { t, i18n } = useTranslation();
+  const { closeChannel } = useNavigationActions();
   const qc = useQueryClient();
   const confirm = useConfirm();
   const [tab, setTab] = useState<"videos" | "about">("videos");
@@ -245,7 +243,7 @@ export default function ChannelPage({
         )}
         <button
           data-testid="channel-back"
-          onClick={onBack}
+          onClick={closeChannel}
           className="absolute top-5 left-7 sm:left-9 z-10 inline-flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm bg-black/45 text-white hover:bg-black/65 backdrop-blur-sm"
         >
           <ArrowLeft className="w-4 h-4" />
@@ -382,7 +380,6 @@ export default function ChannelPage({
             ytSearch={null}
             onYtSearch={() => {}}
             onExitYtSearch={() => {}}
-            onOpenChannel={onOpenChannel}
             channelScoped
           />
           {!ch?.subscribed && nextToken && (