feat(player): watched controls, compact layout, description popover

- Watched: an explicit toggle in the modal (Mark watched / Watched→unmark) plus
  auto-mark when playback reaches the end (within 10s, or on the ended event).
- Compact layout: drop the header bar and the redundant 'Open on YouTube' button
  (the embed's own YouTube logo already jumps out); Close moves to the title row,
  channel + meta share one line — fits without a scrollbar at higher zoom.
- Card actions reflect status: watched shows a double-check, saved a filled
  bookmark, with matching tooltips.
- Description: new GET /api/videos/{id} exposes the already-stored description,
  shown in a popover when hovering the modal title (fetched lazily).

frontend/src/lib/api.ts

@@ -47,6 +47,12 @@ export interface Video {
   watch_url: string;
 }
 
+export interface VideoDetail {
+  id: string;
+  description: string | null;
+  like_count: number | null;
+}
+
 export interface FeedResponse {
   items: Video[];
   has_more: boolean;
@@ -214,6 +220,7 @@ export const api = {
     req(`/api/feed/count?${feedQuery(f, 0, 0)}`),
   setState: (id: string, status: string) =>
     req(`/api/videos/${id}/state`, { method: "POST", body: JSON.stringify({ status }) }),
+  videoDetail: (id: string): Promise<VideoDetail> => req(`/api/videos/${id}`),
   pauseSync: () => req("/api/sync/pause", { method: "POST" }),
   resumeSync: () => req("/api/sync/resume", { method: "POST" }),
   savePrefs: (p: Record<string, any>) =>