feat: tag chips count their channels, Plex shows a file's size, a skip waits, Continue watching opens whole

The channel manager's tag chips carry a muted count. A Plex item's Info page names the file's size.
An auto-skip's countdown starts 3 s into the marker, with its button there from the start and a delay of
0 still immediate. Continue watching has a See all, opening Browse on in progress, last watched first;
Recently added's sets the watch state back so it opens what it says.

frontend/src/lib/api/plex.ts

@@ -154,6 +154,7 @@ export interface PlexItemDetail {
   summary?: string | null;
   year?: number | null;
   duration_seconds: number | null;
+  size_bytes?: number | null; // the file's, null while unknown
   playable: string; // direct | remux | transcode
   transcode_hw?: boolean | null; // transcode items only: true = HW-capable, false = SW-only, null = n/a
   codec_video?: string | null; // e.g. "hevc" / "h264" (info-chip subtitle)