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).

src-tauri/ui/index.html

@@ -15,7 +15,7 @@
     // Frontend host (Tauri asset page). KHÔNG location.href redirect: chuyển
     // sang http://127.0.0.1:8000 sẽ rời Tauri context -> mất window.__TAURI__
     // -> NativeBridgeService vô hiệu (bridge-audio/load_instrument không dùng
-    // được; MIDI rơi về fallback WASM/Carla). Thay vào đó: quét port engine
+    // được; MIDI rơi về fallback WASM/native). Thay vào đó: quét port engine
     // (8000-8010), nạp toàn bộ UI engine qua document.write trên CÙNG document
     // (origin tauri:// giữ nguyên -> __TAURI__ sống), base href + API_BASE_URL
     // trỏ engine HTTP (CORS * đã bật).