Add a STUN responder to the proxy for screen sharing between browsers

The browser build uses it automatically, so no outside server is contacted.
Covered by unit tests and a two-browser step in the web E2E.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/e2e/web-shell.cjs

@@ -1,5 +1,5 @@
 // A bare browser window for the web E2E: no preload, so the page runs as it would in a browser.
-const { app, BrowserWindow, session } = require('electron');
+const { app, BrowserWindow, desktopCapturer, session } = require('electron');
 const fs = require('node:fs');
 const path = require('node:path');
 
@@ -11,6 +11,12 @@ app.whenReady().then(() => {
     item.setSavePath(file);
     item.once('done', (_ev, state) => { if (state === 'completed') fs.writeFileSync(file + '.done', ''); });
   });
+  // Stands in for the browser's own "choose what to share" dialog: always the first screen
+  session.defaultSession.setDisplayMediaRequestHandler(async (_request, callback) => {
+    let sources = [];
+    for (let i = 0; i < 10 && !sources.length; i++) sources = await desktopCapturer.getSources({ types: ['screen'] });
+    try { callback(sources.length ? { video: sources[0] } : {}); } catch { /* refused */ }
+  });
   const win = new BrowserWindow({ width: 1280, height: 800, webPreferences: { contextIsolation: true, sandbox: true, nodeIntegration: false } });
   win.loadURL(process.env.MUMH5_WEB_URL);
 });