feat: native folder picker (Explorer) + Synth liet ke VSTi da scan + fix VU leak (ban 1.1.4)

1) Folder picker dung WINDOW EXPLORER (khong prompt nhap tay):
   - src-tauri/src/lib.rs: IPC bridge — thread watcher ipc/pick_dir.request
     -> run_on_main_thread -> dialog().file().blocking_pick_folder()
     (tauri-plugin-dialog = IFileDialog/Explorer) -> ghi pick_dir.response;
     ghi marker tauri_bridge_ready luc setup.
   - app/api/v1/plugins.py: POST /pick-dir (def sync -> threadpool, khong
     auth) — uu tien Tauri bridge, fallback PowerShell FolderBrowserDialog
     (Win) / osascript (macOS) / zenity-kdialog (Linux).
   - app.jsx pickPluginFolder: 1) pickPluginDir (native) -> 2) __TAURI__
     invoke -> 3) in-app browser -> 4) prompt (cuoi cung).

2) Nut Synth liet ke VSTi da scan (truoc day rong):
   - Root: list_available() chi quet settings.VST_DIR (mac dinh
     /opt/daw_engine/vst3) trong khi Plugin Manager scan plugin_dirs user.
   - plugins.py list_plugins: gop _scan_vst_in_dirs(plugin_dirs) (file
     .vst3/.dll/.so + folder X.vst3 Windows).
   - vst_engine.py: PluginManager.extra_vst_dirs + _scan_plugins quet them
     (ca folder .vst3) + get_plugin_manager doc plugin_dirs.json -> load_vst
     tim thay plugin user scan khi render.

3) VU meter leak: section-tab play -> sang MAIN SESSION -> track MAIN van
   animate theo am section.
   - Root: VU tick fallback _sub_ (sub-node section co analyser) chay cho
     ca canvas MAIN; startSubTabPlayback ghi node o key track MAIN.
   - Fix: fallback _sub_ chi ap dung cho canvas SECTION (isSessVu); 2 trigger
     piano-roll them prefix _sess_ theo st.parent_tab_id.

Verify: 86 tests pass; engine frozen STARTUP 1.35s, 1 engine, 0 spawn '-c';
pick-dir IPC mock tra dung path; scan VST user dirs OK.

app/static/js/services/api.js

@@ -67,6 +67,9 @@ window.API_BASE_URL = window.API_BASE_URL || window.location.origin;
         getPluginDirs: () => apiRequest('/api/v1/plugins/dirs', { method: 'GET' }),
         savePluginDirs: (dirs) => apiRequest('/api/v1/plugins/dirs', { method: 'POST', body: JSON.stringify(dirs) }),
         scanPluginDirs: () => apiRequest('/api/v1/plugins/scan', { method: 'POST' }),
+        // Native folder picker (Explorer qua Tauri bridge / PowerShell) —
+        // user yêu cầu dùng window explorer, không nhập tay
+        pickPluginDir: () => apiRequest('/api/v1/plugins/pick-dir', { method: 'POST' }),
         // Folder picker cho Plugin Manager: duyet thu muc qua backend (media)
         // — hoat dong moi OS, khong can window.__TAURI__ (UI chay tren localhost:8000)
         browseComputer: () => apiRequest('/api/v1/media/computer', { method: 'GET' }),