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/lib/player/gain.test.ts

@@ -2,7 +2,7 @@ import { describe, expect, test } from 'vitest';
 import { dbToLinear, gainDb, playingAsAlbum, MAX_BOOST_DB } from './gain';
 import type { NormalizationPrefs } from '#lib/api/normalization.js';
 import type { TrackRef } from '#lib/api/types.js';
-import { makeTrack } from '$test-utils/fixtures/track';
+import { makeTrack } from '#test-utils/fixtures/track.js';
 
 const prefs = (over: Partial<NormalizationPrefs> = {}): NormalizationPrefs => ({
   mode: 'auto',