The <webview> tag had issues with event listener attachment and React
ref handling, causing the loading overlay to stay forever (white screen).
Switched to a standard <iframe> which is simpler and works reliably:
- Uses iframe onLoad/onError instead of webview dom-ready events
- Added 5s fallback timeout to dismiss loading overlay
- The X-Frame-Options/CSP header overrides from session.webRequest
allow the iframe to load the Control UI