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>

eslint.config.ts

@@ -1095,7 +1095,7 @@ export default defineConfig([
     },
   },
   {
-    files: ['**/*.test.ts', 'web_src/js/test/setup.ts'],
+    files: ['**/*.test.ts', 'web_src/js/vitest.setup.ts'],
     plugins: {vitest},
     languageOptions: {globals: globals.vitest},
     rules: {