Fixes#491
When `--start-maximized` or `--window-size` is passed as a browser arg, Playwright's default viewport (1280x720) overrides the browser's own window sizing, making those flags have no effect on the page content.
This change auto-detects those args and sets `viewport: null` so Playwright defers to the browser's window size. Explicit viewport values still take priority.
Also allows `viewport: null` in the launch protocol for agents that want to disable viewport emulation directly.