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/VideoCard.tsx

@@ -1,5 +1,6 @@
 import { memo, useEffect, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
+import { useNavigationActions } from "./NavigationProvider";
 import {
   Bookmark,
   Check,
@@ -368,7 +369,6 @@ function VideoCard({
   onState,
   onResetState,
   onToggleSave,
-  onOpenChannel,
   onOpen,
 }: {
   video: Video;
@@ -378,10 +378,10 @@ function VideoCard({
   onState: (id: string, status: string) => void;
   onResetState?: (id: string) => void;
   onToggleSave: (id: string, saved: boolean) => void;
-  onOpenChannel?: (channelId: string, channelName: string) => void;
   onOpen?: (v: Video, startAt?: number | null) => void;
 }) {
   const { t, i18n } = useTranslation();
+  const { openChannel } = useNavigationActions();
   const spec = FEED_VIEW_SPEC[view];
   const watched = video.status === "watched";
   // The thumbnail-less row drops Thumb, and with it the duration / live state / saved marker that
@@ -451,7 +451,7 @@ function VideoCard({
       data-testid="video-card-channel"
       onClick={(e) => {
         e.stopPropagation();
-        onOpenChannel?.(video.channel_id, video.channel_title ?? t("card.thisChannel"));
+        openChannel(video.channel_id, video.channel_title ?? t("card.thisChannel"));
       }}
       title={channelClamped ? video.channel_title ?? undefined : undefined}
       className="text-sm text-muted truncate block w-fit max-w-full text-left hover:text-fg"