test: add Vitest infrastructure (config, setup, mocks, fixtures)

Adds test harness for the suite:
- vitest.config.ts: happy-dom env, @/* alias, v8 coverage with 70% thresholds
- setup.ts: env vars, global next/headers and next/navigation mocks
- tsconfig.json: IDE alias resolution for test/
- __mocks__/prisma.ts: centralised Prisma mock auto-registered via vi.mock
- fixtures/users.ts, fixtures/orders.ts: typed test data

test/fixtures/users.ts

@@ -0,0 +1,27 @@
+export const mockUser = {
+  id: 'user-1',
+  email: 'test@example.com',
+  name: 'Test User',
+  passwordHash: '$2a$12$hashedpassword',
+  role: 'CUSTOMER' as const,
+  mustChangePassword: false,
+  createdAt: new Date('2024-01-01'),
+  updatedAt: new Date('2024-01-01'),
+}
+
+export const mockAdmin = {
+  ...mockUser,
+  id: 'admin-1',
+  email: 'admin@example.com',
+  name: 'Admin User',
+  role: 'ADMIN' as const,
+}
+
+export const mockSession = {
+  id: 'session-1',
+  userId: mockUser.id,
+  tokenHash: 'abc123hash',
+  expiresAt: new Date(Date.now() + 30 * 24 * 60 * 60 * 1000),
+  createdAt: new Date('2024-01-01'),
+  user: mockUser,
+}