FEAT: Plugins Manager - danh sach thu muc da muc (Add Directory + x xoa), scan phan loai rieng re VST/SoundFont theo dir

- Backend: plugin_dirs (list) thay vst_dir/soundfont_dir rieng — luu plugin_dirs.json, backward compat gop field cu; scan walk tung dir phan loai .vst3/.dll/.so -> vst_found, .sf2/.sf3 -> soundfonts (kem dir goc)
- SoundFontAutoScanner: system_sf_dirs (list) — scan tat ca thu muc user khai bao
- Frontend: PluginManagerModal — nut Add Directory (Tauri dialog / prompt fallback), moi dir co nut x xoa, nut Scan -> hien danh sach rieng re VST Instruments + SoundFonts (kem duong dan thu muc)
- Tests: +2 (save/get/scan phan loai, xoa dir)

app/core/soundfont_scanner.py

@@ -19,8 +19,11 @@ def _file_sig(path: str) -> tuple:
 
 
 class SoundFontAutoScanner:
-    def __init__(self, system_sf_dir=SYSTEM_SF_DIR, upload_sf_dir=UPLOAD_SF_DIR):
-        self.system_sf_dir = system_sf_dir
+    def __init__(self, system_sf_dir=SYSTEM_SF_DIR, upload_sf_dir=UPLOAD_SF_DIR,
+                 system_sf_dirs=None):
+        # system_sf_dirs (list) — nhiều thư mục user khai báo trong Plugins
+        # Manager. Fallback system_sf_dir (env/.env) nếu list rỗng.
+        self.system_sf_dirs = [d for d in (system_sf_dirs or []) if d] or [system_sf_dir]
         self.upload_sf_dir = upload_sf_dir
         self._catalog = {}
         self._lock = threading.Lock()
@@ -50,6 +53,15 @@ class SoundFontAutoScanner:
                 out.append((fname, os.path.join(directory, fname)))
         return out
 
+    def _all_sf_files(self) -> list:
+        """Gộp file .sf2/.sf3 từ TẤT CẢ thư mục hiệu lực (user dirs + upload)."""
+        out = []
+        for d in self.system_sf_dirs:
+            out.extend(self._sf_files(d))
+        if self.upload_sf_dir and os.path.isdir(self.upload_sf_dir):
+            out.extend(self._sf_files(self.upload_sf_dir))
+        return out
+
     def _inspect_single(self, fname: str, full: str, inspector) -> dict:
         if fname.lower().endswith(".sf2"):
             sf_info = inspector.inspect_sf2_file(full) or {}
@@ -68,9 +80,9 @@ class SoundFontAutoScanner:
 
     def scan_once(self) -> bool:
         from app.core.soundfont_inspector import SoundFontInspector
-        inspector = SoundFontInspector(self.system_sf_dir, self.upload_sf_dir)
+        inspector = SoundFontInspector(self.system_sf_dirs[0], self.upload_sf_dir)
         found_new = False
-        dirs = [(self.system_sf_dir, "system")]
+        dirs = [(d, "system") for d in self.system_sf_dirs]
         if self.upload_sf_dir and os.path.isdir(self.upload_sf_dir):
             dirs.append((self.upload_sf_dir, "upload"))