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
