fix: single-track active mode — only one button red at a time

activePlayTrackIds changed from Set to single trackId string.
All buttons default gray. Click toggles red on one track only.

app/static/js/app.jsx

@@ -5064,7 +5064,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
     }
     var layers = [];
     ghostLayers.forEach(function(layer) {
-      if (activePlayTrackIds !== null && !activePlayTrackIds.has(layer.track_id)) return;
+      if (activePlayTrackIds !== null && activePlayTrackIds !== layer.track_id) return;
       var trk = (activeTracks || []).find(function(t) { return t.id === layer.track_id; });
       layers.push({
         trackId: layer.track_id,
@@ -6142,17 +6142,13 @@ const beatSec = 60.0 / (parseInt(bpm) || 120);
         seenTracks[m._trackId] = true;
         var track = (activeTracks || []).find(function(t) { return t.id === m._trackId; });
         var isActive = m._trackId === st.trackId && m.id === st.target_id;
-        var isPlayOn = activePlayTrackIds === null || activePlayTrackIds.has(m._trackId);
+        var isPlayOn = activePlayTrackIds === m._trackId;
         els.push(React.createElement("button", {
           key: m._trackId,
           onClick: function() {
             if (isActive) return;
             setActivePlayTrackIds(function(prev) {
-              var allIds = allMidiItems.map(function(x) { return x._trackId; });
-              var s = prev === null ? new Set(allIds) : new Set(prev);
-              if (s.has(m._trackId)) s.delete(m._trackId);
-              else s.add(m._trackId);
-              return s.size === allIds.length ? null : s;
+              return prev === m._trackId ? null : m._trackId;
             });
           },
           className: "flex items-center justify-center h-[20px] border border-zinc-600 rounded-md cursor-pointer outline-none mx-1 my-[2px] " + (isActive ? 'bg-yellow-600 text-black font-bold' : (isPlayOn ? 'bg-red-700 text-white font-semibold' : 'bg-zinc-700 text-zinc-300 hover:bg-zinc-600'))