feat(phase2): integrate flow engine and add flows navigation

- Add FLOW_ENGINE_URL to API Gateway config
- Integrate flow engine in message handler (BOT status only)
- Add /flows routes to MainLayout with FlowList and FlowBuilder
- Add Flujos menu item with ApartmentOutlined icon

This completes Phase 2: Flow Engine Básico

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/api-gateway/app/core/config.py

@@ -18,6 +18,9 @@ class Settings(BaseSettings):
     # WhatsApp Core
     WHATSAPP_CORE_URL: str = "http://localhost:3001"
 
+    # Flow Engine
+    FLOW_ENGINE_URL: str = "http://localhost:8001"
+
     # CORS
     CORS_ORIGINS: str = "http://localhost:5173,http://localhost:3000"