CUB-126: Update Control Center deployment for Go + React

- Updated docker-compose.yml for Go + React + PostgreSQL
- Go backend multi-stage Dockerfile (already existed)
- React frontend multi-stage Dockerfile with nginx SPA config (already existed)
- Kiosk start script and systemd unit
- Deployment README
- .env.example for environment variables

.env.example

@@ -0,0 +1,45 @@
+# Control Center - Environment Variables
+# ======================================
+
+# ── Backend Variables ───────────────────────────────────────────────────
+# Server configuration
+PORT=8080
+CORS_ORIGIN=http://localhost:3000
+LOG_LEVEL=info
+ENVIRONMENT=development
+
+# Database connection (PostgreSQL DSN)
+# Format: postgresql://user:password@host:port/database?sslmode=disable
+DATABASE_URL=postgresql://controlcenter:controlcenter@localhost:5432/controlcenter?sslmode=disable
+
+# Gateway (OpenClaw) connection
+# URL to the OpenClaw gateway API for polling agent states
+GATEWAY_URL=http://localhost:18789/api/agents
+# Polling interval for agent state updates
+GATEWAY_POLL_INTERVAL=5s
+
+# ── Frontend Variables (via Vite) ───────────────────────────────────────
+# The Vite config exposes these as import.meta.env.VITE_*
+# Set via environment variable when building: VITE_API_URL
+# VITE_API_URL=http://localhost:8080
+
+# ── Docker Compose Specific ─────────────────────────────────────────────
+# When using docker-compose, these are set in the services section
+# See docker-compose.yml for service-specific environment variables
+
+# ── Database Configuration ─────────────────────────────────────────────
+# Set in the db service environment section of docker-compose.yml
+# POSTGRES_USER=controlcenter
+# POSTGRES_PASSWORD=controlcenter
+# POSTGRES_DB=controlcenter
+
+# ── Development Notes ───────────────────────────────────────────────────
+# For local development without Docker:
+# 1. Start PostgreSQL locally
+# 2. Run: go run ./cmd/server/main.go
+# 3. Run: npm run dev in frontend/
+#
+# For Docker deployment:
+# 1. Copy .env.example to .env (backend only)
+# 2. Run: docker compose up -d
+# 3. Access frontend at http://localhost:3000