chore: set up vitest and add zod

mvp/b2c/vitest.config.ts

@@ -0,0 +1,17 @@
+import { defineConfig } from 'vitest/config';
+import { fileURLToPath } from 'node:url';
+
+export default defineConfig({
+  resolve: {
+    alias: { '@': fileURLToPath(new URL('./src', import.meta.url)) },
+  },
+  test: {
+    environment: 'node',
+    include: ['tests/**/*.test.ts'],
+    coverage: {
+      provider: 'v8',
+      include: ['src/budget/**'],
+      thresholds: { lines: 70, functions: 70, statements: 70, branches: 70 },
+    },
+  },
+});