test(web): add Vitest setup and unit tests for validations and utils

- Add vitest config and test script to web app
- Auth validation tests: phone format, password rules, registration flow
- Listing validation tests: all schema steps, constants, merged schema
- Utils tests: cn() class merging with Tailwind conflict resolution
- 36 tests across 3 test files

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/package.json

@@ -7,6 +7,7 @@
     "build": "next build",
     "start": "next start",
     "lint": "next lint",
+    "test": "vitest run",
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
@@ -34,6 +35,7 @@
     "postcss": "^8.4.0",
     "tailwindcss": "^3.4.0",
     "tailwindcss-animate": "^1.0.7",
-    "typescript": "^6.0.2"
+    "typescript": "^6.0.2",
+    "vitest": "^4.1.3"
   }
 }