fix: AI tracks auto-align to end of last existing item

- Replace currentTime with auto-calculated maxEnd from existing items
- Each AI generation starts right after the previous one
- Ensures tracks from different generations align in bar columns

app/static/js/app.jsx

@@ -13854,7 +13854,17 @@ const App = () => {
               };
               updatedTracks.push(targetTrack);
             }
-            const itemStartTimeSec = currentTime;
+            const itemStartTimeSec = (() => {
+              // Auto-align: place new AI tracks right after the last existing item
+              let maxEnd = 0;
+              updatedTracks.forEach(t => {
+                (t.midiItems || []).forEach(mi => {
+                  const miEnd = (mi.startTime || 0) + (mi.duration || 0);
+                  if (miEnd > maxEnd) maxEnd = miEnd;
+                });
+              });
+              return maxEnd > 0 ? maxEnd : 0;
+            })();
             const newMidiItem = {
               id: 'item_ai_' + Date.now() + '_' + Math.random().toString(36).substr(2, 5),
               name: `${composition_title || 'AI Theme'} - ${aiTrack.track_name}`,