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/vitest.config.ts

@@ -0,0 +1,36 @@
+import { defineConfig } from 'vitest/config'
+import path from 'path'
+
+export default defineConfig({
+  test: {
+    environment: 'happy-dom',
+    globals: true,
+    setupFiles: [path.resolve(__dirname, 'setup.ts')],
+    include: [
+      '../test/unit/**/*.{test,spec}.{ts,tsx}',
+      '../test/integration/**/*.{test,spec}.{ts,tsx}',
+      '../test/components/**/*.{test,spec}.{ts,tsx}',
+    ],
+    exclude: ['**/node_modules/**'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'html', 'lcov'],
+      include: ['src/**/*.ts', 'src/**/*.tsx'],
+      exclude: [
+        'src/app/layout.tsx',
+        'src/lib/prisma.ts',
+        'src/**/*.d.ts',
+      ],
+      thresholds: { lines: 70, functions: 70 },
+    },
+  },
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, '../app/src'),
+    },
+  },
+  esbuild: {
+    jsx: 'automatic',
+    jsxImportSource: 'react',
+  },
+})