[DevTools] Fix XSS in standalone onError via innerHTML

[tomohiro86]

Summary

In packages/react-devtools-core/src/standalone.js, the onError function was interpolating a server error message string directly into an innerHTML template literal:

node.innerHTML = `
  <div class="box-content">
    ${message}
  </div>
`;

If the message string contained HTML (e.g. <img src=x onerror="...">) it would be parsed and executed by the browser.

This PR fixes it by setting the static HTML structure first, then assigning the message via textContent, which the browser treats as plain text and never interprets as markup.

node.innerHTML = `
  <div class="box">
    <div class="box-header">Unknown error</div>
    <div class="box-content"></div>
  </div>
`;
const contentNode = node.querySelector('.box-content');
if (contentNode !== null) {
  contentNode.textContent = message;
}

The message value comes from Node.js http.Server / net.Server error events (server.on('error', ...)). These are normally OS/network-level error strings and not directly attacker-controlled, so real-world exploitability is limited. Nevertheless the code structure allowed HTML injection and the fix is straightforward.

How did you test this change?

Manually verified that a normal server error (e.g. EACCES) still renders the error message as plain text in the standalone DevTools shell. No automated tests exist for this UI path in react-devtools-core.

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