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>

types.d.ts

@@ -3,7 +3,7 @@ declare module '*.svg' {
   export default value;
 }
 
-declare module '*.css' {
+declare module '*.txt' {
   const value: string;
   export default value;
 }