Screen sharing: resizable tile stage, streams first, pop-out window

- The stage above the chat is as tall as the user drags it; tiles are sized to
  fill it, with a meeting-style look
- Tiles with a stream come first, at the top left
- The large picture can be moved to a separate window

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

electron/main.ts

@@ -201,7 +201,11 @@ function createWindow() {
   });
 
   // Links from chat open in the system browser, never inside the app
-  win.webContents.setWindowOpenHandler(({ url }) => {
+  win.webContents.setWindowOpenHandler(({ url, frameName }) => {
+    // The stream window: an empty page the app fills with the video itself
+    if (frameName === 'mumh5-stream' && (url === 'about:blank' || url === '')) {
+      return { action: 'allow', overrideBrowserWindowOptions: { width: 960, height: 600, minWidth: 320, minHeight: 200, backgroundColor: '#000000', autoHideMenuBar: true, title: 'mumh5' } };
+    }
     if (/^https?:\/\//i.test(url)) shell.openExternal(url);
     return { action: 'deny' };
   });