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/utils/match.test.ts

@@ -1,9 +1,7 @@
 import {GET} from '../modules/fetch.ts';
 import {matchEmoji, matchMention} from './match.ts';
 
-vi.mock('../modules/fetch.ts', () => ({
-  GET: vi.fn(),
-}));
+vi.mock('../modules/fetch.ts', () => ({GET: vi.fn()}));
 
 const testMentions = [
   {key: 'user1 User 1', value: 'user1', name: 'user1', fullname: 'User 1', avatar: 'https://avatar1.com'},