fix: nut Synth tren TCP khong hien danh sach VSTi da scan (ban 1.1.5)

- Nut Synth TCP goi openInstrumentSelector (modal 'Select Instrument')
  nhung modal chi co category SoundFont + GM - khong co muc VST (muc VST
  chi ton tai o dropdown MixerStrip). Them 'VST Instruments' vao sidebar
  trai modal: list instrumentSelectorData.vst_instruments (listPlugins),
  click -> setTrackInstrumentWithUndo(trackId, v.id, v.name) + dong modal.
- _scan_vst_in_dirs + /scan: phat hien ca FOLDER ten X.vst3 (Windows VST3
  = folder chua X.vst3.dll) - truoc day chi scan file.
- Ghi chu: pedalboard (engine VST) la headless - KHONG mo duoc GUI goc cua
  plugin; danh sach + chon la co che chon nhac cu kha thi.

Verify engine frozen: /plugins/available tra ['PadMachine','UltraSynth']
(ca file va folder .vst3); /scan dong bo; test_plugin_api 3 passed.

app/api/v1/plugins.py

@@ -111,7 +111,14 @@ async def scan_plugin_dirs(background_tasks: BackgroundTasks = None,
     for d in plugin_dirs:
         if not os.path.isdir(d):
             continue
-        for root, _dirs, files in os.walk(d):
+        for root, dirs, files in os.walk(d):
+            # Windows: VST3 là FOLDER tên X.vst3 (chứa X.vst3.dll bên trong)
+            for sub in list(dirs):
+                if sub.lower().endswith(".vst3"):
+                    vst_found.append({"name": os.path.splitext(sub)[0],
+                                      "path": os.path.join(root, sub),
+                                      "dir": d,
+                                      "type": "VST3"})
             for f in files:
                 low = f.lower()
                 if low.endswith(".vst3") or low.endswith(".dll") or low.endswith(".so"):
@@ -170,12 +177,22 @@ async def list_plugins(current_user: dict = Depends(get_current_user)):
 
 def _scan_vst_in_dirs(dirs: list) -> list:
     """Walk các thư mục (plugin_dirs user) → danh sách VST giống /scan:
-    file .vst3/.dll/.so (VST3 folder Windows = .vst3.dll bên trong)."""
+    file .vst3/.dll/.so + FOLDER tên X.vst3 (Windows VST3 = folder chứa
+    X.vst3.dll bên trong)."""
     found = {}
     for d in dirs:
         if not d or not os.path.isdir(d):
             continue
-        for root, _dirs, files in os.walk(d):
+        for root, dirs, files in os.walk(d):
+            # Windows: VST3 là FOLDER tên X.vst3
+            for sub in list(dirs):
+                if sub.lower().endswith(".vst3"):
+                    name = os.path.splitext(sub)[0]
+                    if name not in found:
+                        found[name] = {
+                            "id": name, "name": name, "type": "VST3",
+                            "path": os.path.join(root, sub),
+                        }
             for f in files:
                 low = f.lower()
                 if low.endswith(".vst3") or low.endswith(".dll") or low.endswith(".so"):