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

package.json

@@ -7,7 +7,9 @@
     "dev": "vite",
     "build": "tsc -b && vite build",
     "lint": "eslint .",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "clsx": "^2.1.1",
@@ -21,6 +23,9 @@
   "devDependencies": {
     "@eslint/js": "^9.39.1",
     "@tailwindcss/vite": "^4.1.18",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
+    "@testing-library/user-event": "^14.6.1",
     "@types/node": "^24.10.1",
     "@types/react": "^19.2.5",
     "@types/react-dom": "^19.2.3",
@@ -29,9 +34,11 @@
     "eslint-plugin-react-hooks": "^7.0.1",
     "eslint-plugin-react-refresh": "^0.4.24",
     "globals": "^16.5.0",
+    "jsdom": "^27.4.0",
     "tailwindcss": "^4.1.18",
     "typescript": "~5.9.3",
     "typescript-eslint": "^8.46.4",
-    "vite": "^7.2.4"
+    "vite": "^7.2.4",
+    "vitest": "^4.0.18"
   }
 }