infra: setup vitest testing framework and initial auth utility test suite (#76)

CONTRIBUTING.md

@@ -253,6 +253,16 @@ We welcome test contributions! While the project doesn't yet have full test cove
 - **Component tests** for complex interactive UI (modals, forms).
 - Place test files alongside the code they test, named `*.test.ts` or `*.test.tsx`.
 
+To run the test suite locally:
+```bash
+npm run test
+```
+
+To run tests in watch mode during development:
+```bash
+npm run test:watch
+```
+
 ---
 
 ## 📬 Submitting a Pull Request

app/utils/auth.server.test.ts

@@ -0,0 +1,28 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { isSessionValid, formatSessionCookie } from './auth.server';
+
+describe('Auth Utilities', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date(2026, 0, 1).getTime());
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it('validates unexpired session correctly', () => {
+    const expiresAt = new Date(2026, 0, 2).getTime();
+    expect(isSessionValid(expiresAt)).toBe(true);
+  });
+
+  it('invalidates expired session correctly', () => {
+    const expiresAt = new Date(2025, 0, 1).getTime();
+    expect(isSessionValid(expiresAt)).toBe(false);
+  });
+
+  it('formats session cookie string correctly', () => {
+    const cookie = formatSessionCookie('abc-123');
+    expect(cookie).toBe('session_id=abc-123; HttpOnly; Secure; Path=/');
+  });
+});

app/utils/auth.server.ts

@@ -0,0 +1,7 @@
+export function isSessionValid(expiresAt: number): boolean {
+  return Date.now() < expiresAt;
+}
+
+export function formatSessionCookie(sessionId: string): string {
+  return `session_id=${sessionId}; HttpOnly; Secure; Path=/`;
+}