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/core/render_engine.py

@@ -227,7 +227,7 @@ def _apply_rebalance(buf, params, sr):
 
 def _apply_builtin_fx_chain(buf, chain, sr):
     """Serial pipeline qua các builtin custom FX slots (spec §2 Step 2: Slot 1..n
-    in-place). Slot inactive/bypass → bỏ qua. vst3/carla slots = xử lý ngoài
+    in-place). Slot inactive/bypass → bỏ qua. vst3 slots = xử lý ngoài
     (bridge) → không thêm DSP ở đây."""
     for slot in chain or []:
         if isinstance(slot, str):