feat(player): a video's related videos, on demand beside it

The player shows what YouTube lists beside the video, without what the
user has watched or hidden. Wide, a Related tab beside the description;
stacked, a button beside the info opens them in a drawer. A row plays in
the same player (Back returns) and the list stays the opened video's.
A switch picks on-topic or Hungarian-trending lists, kept on the account.

Each related row says whether the user follows its channel; the row is
the video card's narrow form with its whole menu. VideoMenu moves to
features/video, where the player can reach it.

frontend/src/lib/queryKeys.ts

@@ -32,6 +32,8 @@ export const qk = {
   ytSearch: (q?: string | null, count?: number) =>
     q !== undefined ? (["yt-search", q, count] as const) : (["yt-search"] as const),
   videoDetail: (id: string) => ["video-detail", id] as const,
+  related: (id?: string, region?: string) =>
+    id !== undefined ? (["related", id, region] as const) : (["related"] as const),
 
   // --- channels ---
   channels: () => ["channels"] as const,