add unit test suite: 80 tests across utils, api, auth, clients, events, emails

- Vitest + React Testing Library + jsdom setup
- utils.test.ts: cn, formatDate, formatFullDate, getInitials, getRelativeTime, getDaysUntil
- api.test.ts: token management, auth, CRUD for clients/events/emails, admin, error handling
- auth.test.ts: login, logout, checkSession, setUser
- clients.test.ts: fetch, create, update, delete, markContacted, filters
- events.test.ts: fetch, create, update, delete, syncAll
- emails.test.ts: fetch, generate, update, send, delete

vitest.config.ts

@@ -0,0 +1,18 @@
+import { defineConfig } from 'vitest/config';
+import react from '@vitejs/plugin-react';
+import path from 'path';
+
+export default defineConfig({
+  plugins: [react()],
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, './src'),
+    },
+  },
+  test: {
+    globals: true,
+    environment: 'jsdom',
+    setupFiles: ['./src/test/setup.ts'],
+    include: ['src/**/*.test.{ts,tsx}'],
+  },
+});