Remove Carla native VSTi bridge entirely

Drop Carla integration across frontend and backend:
- plugins.py: remove /carla-midi, /carla-status, /carla-stop, /open-in-carla,
  /carla-play-notes endpoints and Carla process management
- runtime.py: remove find_carla / _carla_* helpers, detect and capabilities
- app.jsx: remove Carla bridge UI, SonicCarlaMidi routing, shouldRouteCarla
  guards, VST row Carla button and preset fallback
- system.py: remove /carla-path
- Keep audio path unchanged (tracks -> masterBus -> output -> analyser),
  directOut only for offline render

Build frontend, tests pass (124 passed).

.env.example

@@ -26,7 +26,7 @@ PIANOBK_DIR=/home/locpham/daw_assets/pianobook
 
 # ── Runtime (tự phát hiện môi trường) ──
 # auto (mặc định): Windows/macOS → desktop; Linux không DISPLAY hoặc docker → headless.
-# desktop: server + client cùng 1 máy (bật nút "Mở trong Carla" nếu có Carla local)
+# desktop: server + client cùng 1 máy (native GUI VSTi mở được trên máy)
 # headless: server docker + browser UI (soundfont + VSTi mở được từ storage mount;
 #           preset chỉnh trên máy khác → upload .vstpreset qua web UI)
 SF_RUNTIME=auto