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/tsconfig.json

@@ -0,0 +1,15 @@
+{
+  "extends": "../app/tsconfig.json",
+  "compilerOptions": {
+    "baseUrl": ".",
+    "paths": {
+      "@/*": ["../app/src/*"]
+    },
+    "types": ["vitest/globals", "@testing-library/jest-dom"]
+  },
+  "include": [
+    "./**/*.ts",
+    "./**/*.tsx"
+  ],
+  "exclude": ["node_modules"]
+}