test(renderer): add failing test for cn() helper + vitest config

tests/unit/lib/cn.test.ts

@@ -0,0 +1,20 @@
+import { describe, it, expect } from 'vitest';
+import { cn } from '@/lib/utils';
+
+describe('cn', () => {
+  it('merges class names', () => {
+    expect(cn('a', 'b')).toBe('a b');
+  });
+
+  it('deduplicates conflicting tailwind classes', () => {
+    expect(cn('p-2', 'p-4')).toBe('p-4');
+  });
+
+  it('ignores falsy values', () => {
+    expect(cn('a', false, null, undefined, '', 'b')).toBe('a b');
+  });
+
+  it('accepts conditional class objects', () => {
+    expect(cn('base', { active: true, disabled: false })).toBe('base active');
+  });
+});