Fix: prevent duplicate mousemove listeners in WebViewPluginDemoGUI

[alex-js-ltd]

Summary

• App previously registered a document mousemove listener directly in the component body (not inside useEffect), so a new listener was added on every re-render without the old one being removed — a listener leak.
• Moves the listener registration into a useEffect with an empty dependency array and adds a cleanup function that removes it on unmount.

Discussed on the forum: https://forum.juce.com/t/event-listener-leak-in-the-webviewplugindemogui/69236

Test plan

• Run the WebView plugin demo GUI and confirm mouse-move driven parameter control still works.
• Verify only one mousemove listener is attached after multiple re-renders (e.g. via getEventListeners(document) in devtools).