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).

app/config.py

@@ -62,7 +62,7 @@ class Settings:
     VST_DIR: str = os.getenv("VST_DIR", _default_vst_dir())
     SOUNDFONT_DIR: str = os.getenv("SOUNDFONT_DIR", _default_soundfont_dir())
 
-    # Thư viện preset VST3 (.vstpreset) — cầu nối Carla → pedalboard.
+    # Thư viện preset VST3 (.vstpreset) — cầu nối preset → pedalboard.
     # Nằm trong storage nên tự động nằm trong volume mount (docker) / thư mục
     # storage (Windows desktop).
     PRESET_DIR: str = os.path.join(STORAGE_DIR, "presets")