test: run frontend unit tests in browsers (#38860)

Run them in headless [vitest browser
mode](https://vitest.dev/guide/browser/) in chromium and firefox.
Similar UX than current tests, it's about 5 times as slow (goes from 1s
to 5s on my machine), but definitely worth it as it removes all
happy-dom problems.

---------

Signed-off-by: silverwind <me@silverwind.io>

web_src/js/vitest.setup.ts

@@ -1,7 +1,7 @@
 import './globals.ts';
 
 window.config = {
-  appUrl: 'http://localhost:3000/',
+  appUrl: `${window.location.origin}/`,
   appSubUrl: '',
   assetUrlPrefix: '/assets',
   sharedWorkerUri: '',