fix: AIPresetModal merge backend presets instead of overwrite

Backend sync effect (useEffect mount) was unconditionally
replacing mgr.presets with data.presets, destroying:
- Frontend-only default presets (12 vs backend's 7)
- User-defined presets in localStorage not yet synced to backend

Fix: merge backend presets by id — keep all local presets,
append any backend presets whose id doesn't exist locally.
User presets + frontend defaults are never lost.

app/static/js/app.jsx

@@ -4308,19 +4308,27 @@ const AIPresetModal = ({ isOpen, onClose }) => {
   const [formScale, setFormScale] = React.useState('C Minor');
   const [formTemplate, setFormTemplate] = React.useState('');
 
-  // Try to sync from backend on mount
+  // Sync from backend on mount — merge into local presets, never overwrite
   React.useEffect(() => {
     if (!window.SonicAPI) return;
     setSyncing(true);
     window.SonicAPI.getAIPresets()
       .then(data => {
-        if (data && data.presets && data.presets.length > 0) {
-          mgr.presets = data.presets;
-          setPresets([...data.presets]);
+        if (!data || !data.presets || data.presets.length === 0) return;
+        var existing = mgr.presets;
+        var existingIds = new Set(existing.map(function(p) { return p.id; }));
+        var merged = existing.slice();
+        data.presets.forEach(function(bp) {
+          if (!existingIds.has(bp.id)) {
+            merged.push(bp);
+            existingIds.add(bp.id);
           }
+        });
+        mgr.presets = merged;
+        setPresets(merged);
       })
-      .catch(() => {})
-      .finally(() => setSyncing(false));
+      .catch(function() {})
+      .finally(function() { setSyncing(false); });
   }, []);
 
   const savePresets = (newPresets) => {