fix(web): user-playlist play omits third arg to satisfy contract test

74bae74f routed per-artist system mixes through getPlaylist and
always passed the source attribution as the third arg, falling
back to undefined for true user playlists. Vitest's toHaveBeenCalledWith
is arity-strict — playQueue(refs, 0, undefined) is not the same as
playQueue(refs, 0) — so the PlaylistCard contract test failed on
the user-playlist case.

Split the call: pass three args only when a variant tag exists,
two args for user playlists. Preserves source attribution for
songs_like_artist and keeps user playlists source-less as the
test pins.

web/src/lib/components/PlaylistCard.svelte

@@ -86,10 +86,17 @@
         const detail = await getPlaylist(playlist.id);
         const refs = toTrackRefs(detail.tracks);
         if (refs.length > 0) {
-          // Tag the queue with the variant when one exists so play
-          // events still carry the correct source attribution even
-          // though we went through the per-id path.
-          playQueue(refs, 0, variant != null ? { source: variant } : undefined);
+          // Per-artist system mixes (songs_like_artist) carry a
+          // variant tag so play_started still attributes the source
+          // correctly even though we routed through the per-id path.
+          // True user playlists (variant == null) call with two args
+          // so source attribution stays absent — the PlaylistCard
+          // test pins this contract.
+          if (variant != null) {
+            playQueue(refs, 0, { source: variant });
+          } else {
+            playQueue(refs, 0);
+          }
         }
       }
     } finally {