feat(channels): an Added column, and About cards that wait for a resting pointer

The channel manager lists when each follow reached the list (the
subscription import that first saw it, or a subscribe made here) as a
sortable Added date after Last upload. The About cell's card opens after the
pointer rests on it for half a second instead of at once, and a scroll puts
an open one away.

frontend/src/lib/api/channels.ts

@@ -73,6 +73,9 @@ export interface ManagedChannel {
   description: string | null;
   stored_videos: number;
   last_video_at: string | null;
+  /** When the follow reached this list: the subscription import that first saw it, or a
+   *  subscribe made here. */
+  added_at: string;
   total_duration_seconds: number;
   priority: number;
   hidden: boolean;