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/vitest.config.ts

@@ -1,7 +1,7 @@
 import { sveltekit } from '@sveltejs/kit/vite';
 import { svelteTesting } from '@testing-library/svelte/vite';
 import { configDefaults, defineConfig } from 'vitest/config';
-import { kitOptions } from './vite.config';
+import { kitOptions } from './vite.config.ts';
 
 export default defineConfig({
   plugins: [sveltekit(kitOptions), svelteTesting()],