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>

server/main.ts

@@ -8,10 +8,11 @@ const config = configFromEnv(process.env);
 config.staticDir ??= [path.resolve(import.meta.dirname, '../dist-web')].find(d => existsSync(path.join(d, 'index.html'))) ?? null;
 
 try {
-  const { port } = await startProxy(config);
+  const { port, stunPort } = await startProxy(config);
   console.log(`mumh5 proxy listening on http://${config.bind}:${port}`);
   console.log(config.allowAny ? `Allowed servers: any${config.allowPrivate ? ', private addresses included' : ' public address'}` : `Allowed servers: ${config.servers.map(s => `${s.host}:${s.port}`).join(', ')}`);
   if (config.sendProxy) console.log('Announcing client addresses with the PROXY protocol; the allowed servers must expect it');
+  console.log(stunPort ? `STUN for screen sharing on UDP port ${stunPort} (must be reachable from the internet)` : 'STUN is off; screen sharing between browser users will only work on the same network');
   console.log(config.staticDir ? `Serving the web app from ${config.staticDir}` : 'No web build found (npm run build:web); serving the API only');
 } catch (e) {
   console.error((e as Error).message);