fix(userfront-e2e): fix widespread test failures in non-Chromium browsers

- Add COOP/COEP headers to serve script for Flutter WASM compatibility (SharedArrayBuffer)
- Update CI workflow to install all Playwright browsers for userfront-e2e
- Fix command reporting consistency in adminfront test script

userfront-e2e/scripts/serve-userfront-build.mjs

@@ -81,6 +81,12 @@ const server = createServer((req, res) => {
   res.setHeader('Last-Modified', stats.mtime.toUTCString());
   res.setHeader('Cache-Control', cacheControl);
   res.setHeader('Vary', 'Accept-Encoding');
+
+  // Flutter WASM requires SharedArrayBuffer which needs these COOP/COEP headers
+  // to be cross-origin isolated in most modern browsers (WebKit, Firefox, etc.)
+  res.setHeader('Cross-Origin-Opener-Policy', 'same-origin');
+  res.setHeader('Cross-Origin-Embedder-Policy', 'require-corp');
+
   if (servedPath === brotliPath) {
     res.setHeader('Content-Encoding', 'br');
   }