fix(web/playlists): attribute source when playing a system playlist from its detail page

Playing a system playlist from /playlists/<id> previously sent no source, so
it never advanced that playlist's rotation — inconsistent with the home tile
(and the Android detail screen, which already tags the variant). Pass
source: variant alongside the existing self-heal closure so a play is
attributed regardless of the surface it started from. Issue #968.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/routes/playlists/[id]/+page.svelte

@@ -70,13 +70,14 @@
     // so the index in the playable list != the playlist position.
     const clickedTrackID = data.tracks.find((t) => t.position === position)?.track_id;
     const startIdx = live.findIndex((t) => t.id === clickedTrackID);
-    // #968: a system playlist played from its detail page gets the same
-    // self-heal closure as the home tile, so a stale snapshot re-pulls on
-    // total failure. (Source attribution is intentionally left to the home
-    // tile / shuffle path — this only adds the recovery closure.)
+    // #968: a system playlist played from its detail page is tagged with its
+    // source (so play_started advances the rotation, matching the home tile and
+    // the Android detail screen) and gets the self-heal closure so a stale
+    // snapshot re-pulls on total failure.
     const variant = data.system_variant;
     if (variant != null) {
       playQueue(live, Math.max(0, startIdx), {
+        source: variant,
         refetch: systemPlaylistRefetch({
           variant,
           playlistId: id,