Test screen logic under happy-dom so filter and error-code mistakes fail in CI.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/HSchool.Client/vite.config.ts

@@ -1,4 +1,4 @@
-import { defineConfig } from 'vite';
+import { defineConfig } from 'vitest/config';
 
 // Aspire injects SERVER_HTTP / SERVER_HTTPS from the `server` resource reference,
 // so the dev server proxies to whatever port the backend actually got.
@@ -24,4 +24,7 @@ export default defineConfig({
     target: 'es2022',
     sourcemap: true,
   },
+  test: {
+    environment: 'node',
+  },
 });