fix(preview): download soundfont resolve plugin_dirs + fallback default GM (hết oscillator)

app/api/v1/plugins.py

@@ -601,6 +601,16 @@ async def download_soundfont_asset(sf_id: str):
                 except Exception as e:
                     print(f"[soundfont-download] SF3->SF2 conversion failed for {full}: {e}")
                 return FileResponse(full, media_type="application/octet-stream", filename="soundfont.sf3")
+    # Not found in the 3 standard dirs -> resolve like offline render
+    # (render_engine._find_sf2_path): scan user plugin_dirs (Downloads...),
+    # then fall back to default GM font. UI preview must hear the SAME
+    # instrument as offline; before, 404 -> soundfontPlayer.js played a
+    # fallback oscillator (synth sound).
+    resolved = _find_sf2_path(clean_id)
+    if not resolved or not os.path.exists(resolved):
+        resolved = _find_default_sf2()
+    if resolved and os.path.exists(resolved):
+        return FileResponse(resolved, media_type="application/octet-stream", filename=os.path.basename(resolved))
     raise HTTPException(status_code=404, detail="SoundFont asset not found")