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.

backend/app/routes/plex/detail.py

@@ -432,6 +432,7 @@ def item_detail(
         "summary": (it.summary_hu or it.summary) if hu else it.summary,
         "year": it.year,
         "duration_seconds": it.duration_s,
+        "size_bytes": it.size_bytes,
         "playable": it.playable,
         # Playback-class detail: HW/SW transcode split + the codec/bit-depth the info-page chip shows.
         "transcode_hw": it.transcode_hw,