feat(player): linkify descriptions and play YouTube links inline

- Narrow the title hover target to the actual text, not the whole row.
- Linkify descriptions: timestamps (mm:ss / hh:mm:ss) seek the player; emails
  become mailto:; hashtags link to YouTube's hashtag feed; other URLs open in a
  new tab. Blank lines are stripped so the popover isn't mostly whitespace.
- YouTube links play in the inline player: a link to the current video seeks
  (honoring t=), a link to another video navigates the player to it, with a Back
  button to the original. While on a linked video the title/author come from the
  player and its views/date/duration + a clickable channel come from the detail
  endpoint, which falls back to the YouTube API (videos.list, attributed to the
  user) for videos not in our DB.

frontend/src/lib/api.ts

@@ -51,6 +51,12 @@ export interface VideoDetail {
   id: string;
   description: string | null;
   like_count: number | null;
+  in_db: boolean;
+  channel_id: string | null;
+  channel_title: string | null;
+  published_at: string | null;
+  view_count: number | null;
+  duration_seconds: number | null;
 }
 
 export interface FeedResponse {