chore(web): clear kit 3's alias and Vite's config-loader deprecations (#5021)

CI run 8789 warned on both:

- kit 3 deprecates `config.alias`. The $test-utils alias becomes a
  `#test-utils/*` subpath import, matching #lib, and its 43 import sites
  move with it.
- Vite's coming native config loader needs the extension on
  vitest.config.ts's import of vite.config.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/routes/library/liked/liked.test.ts

@@ -4,7 +4,7 @@ import { mockInfiniteQuery } from '../../../test-utils/query';
 import { apiClientMock } from '../../../test-utils/mocks/client';
 import type { ArtistRef, AlbumRef, TrackRef, Page } from '#lib/api/types.js';
 import { readable } from 'svelte/store';
-import { makeTrack } from '$test-utils/fixtures/track';
+import { makeTrack } from '#test-utils/fixtures/track.js';
 
 vi.mock('#lib/api/likes.js', () => ({
   createLikedIdsQuery: () => readable({