feat: Add configurable port for openhands serve

[Chesars]

Summary

Fixes #779

Adds a --port option to openhands serve so users can access the OpenHands GUI container on a custom host port.

The Docker container still listens on port 3000; only made the host-side Docker port configurable.

Example:

openhands serve --port 4000

Docker runs with: -p 4000:3000

Testing

• uv run pytest tests/test_main.py::test_main_serve_calls_launch_gui_server tests/test_gui_launcher.py::TestLaunchGuiServer::test_launch_gui_server_scenarios
• Manually verified openhands serve --port 4000 in Docker as 0.0.0.0:4000->3000/tcp