fix(playback): give the now-playing "source" button a per-track context

The "playing from" button read a single global context set once at play time;
refresh_now_playing_meta never republished it, so it went stale across track
changes (previous track's album) and single-track plays fell back to the
current track's album instead of the playlist it came from. Add per-track
context_kind/context_id to QueueTrack (serde-default None, back-compat), stamp
the real container at every play path (album/artist/playlist/label), and
republish the context from the current track on EVERY track change (gapless
included) — never cached. open-context routes all stamped kinds.

cc

crates/qbz-player/src/queue.rs

@@ -1258,6 +1258,8 @@ mod tests {
             source: Some("test".to_string()),
             parental_warning: false,
             source_item_id_hint: None,
+            context_kind: None,
+            context_id: None,
         }
     }