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/components/FlagPopover.test.ts

@@ -1,7 +1,7 @@
 import { afterEach, describe, expect, test, vi } from 'vitest';
 import { render, screen, fireEvent } from '@testing-library/svelte';
 import FlagPopover from './FlagPopover.svelte';
-import { makeTrack } from '$test-utils/fixtures/track';
+import { makeTrack } from '#test-utils/fixtures/track.js';
 
 const invalidateMock = vi.fn();
 vi.mock('@tanstack/svelte-query', async (orig) => {