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/tests/test_plex_detail.py

@@ -219,6 +219,7 @@ def test_item_detail_movie_serves_from_row_despite_live_outage(db, monkeypatch,
         library_id=lib.id,
         kind="movie",
         title="The Godfather",
+        size_bytes=15_000_000_000,
         genres=["Crime", "Drama"],
         directors=["Francis Ford Coppola"],
         studio="Paramount",
@@ -238,6 +239,7 @@ def test_item_detail_movie_serves_from_row_despite_live_outage(db, monkeypatch,
     resp = _get(db, user, monkeypatch, override_dep, "/api/plex/item/m1")
     assert resp.status_code == 200, resp.text
     body = resp.json()
+    assert body["size_bytes"] == 15_000_000_000
     assert body["imdb_rating"] == 8.7
     assert body["imdb_id"] == "tt0068646"
     assert body["imdb_url"] == "https://www.imdb.com/title/tt0068646/"