Add structured logging system with color-coded output and file logging

- Add comprehensive logging system with color-coded log levels
- Wrap all command executions with run_cmd() to capture output
- Add file logging with timestamps (default: openvpn-install.log)
- Suppress interactive prompts in auto-install mode
- Show log file location hint on errors
- Add E2E output validation to catch raw echo leaks

docker-compose.yml

@@ -5,7 +5,7 @@ services:
       context: .
       dockerfile: test/Dockerfile.server
       args:
-        BASE_IMAGE: ${BASE_IMAGE:-}
+        BASE_IMAGE: ${BASE_IMAGE:-ubuntu:24.04}
     container_name: openvpn-server
     hostname: openvpn-server
     cap_add: