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/__mocks__/prisma.ts

@@ -0,0 +1,81 @@
+import { vi } from 'vitest'
+
+export const prisma = {
+  user: {
+    findUnique: vi.fn(),
+    findMany: vi.fn(),
+    create: vi.fn(),
+    update: vi.fn(),
+    delete: vi.fn(),
+    count: vi.fn(),
+  },
+  session: {
+    create: vi.fn(),
+    findUnique: vi.fn(),
+    delete: vi.fn(),
+    deleteMany: vi.fn(),
+  },
+  loginAttempt: {
+    count: vi.fn(),
+    create: vi.fn(),
+    deleteMany: vi.fn(),
+  },
+  order: {
+    create: vi.fn(),
+    update: vi.fn(),
+    findUnique: vi.fn(),
+    findMany: vi.fn(),
+    count: vi.fn(),
+  },
+  orderItem: {
+    create: vi.fn(),
+    findMany: vi.fn(),
+  },
+  payment: {
+    create: vi.fn(),
+    update: vi.fn(),
+    updateMany: vi.fn(),
+    findFirst: vi.fn(),
+    findMany: vi.fn(),
+  },
+  product: {
+    findUnique: vi.fn(),
+    findMany: vi.fn(),
+    create: vi.fn(),
+    update: vi.fn(),
+    delete: vi.fn(),
+    count: vi.fn(),
+  },
+  productType: {
+    findUnique: vi.fn(),
+    findMany: vi.fn(),
+    create: vi.fn(),
+    update: vi.fn(),
+    delete: vi.fn(),
+  },
+  category: {
+    findUnique: vi.fn(),
+    findMany: vi.fn(),
+    create: vi.fn(),
+    update: vi.fn(),
+    delete: vi.fn(),
+  },
+  review: {
+    findMany: vi.fn(),
+    create: vi.fn(),
+    update: vi.fn(),
+    delete: vi.fn(),
+    count: vi.fn(),
+  },
+  auditLog: {
+    create: vi.fn(),
+    findMany: vi.fn(),
+  },
+  siteSettings: {
+    findMany: vi.fn(),
+    upsert: vi.fn(),
+  },
+  $transaction: vi.fn((fn: (tx: unknown) => unknown) => fn(prisma)),
+}
+
+vi.mock('@/lib/prisma', () => ({ prisma }))