feat: implement user registration functionality

- Created registerUser server action with email/password validation
- Implemented password hashing with bcrypt (10 rounds)
- Added email uniqueness check to prevent duplicate registrations
- Created session after successful registration using NextAuth
- Added comprehensive validation for email format and password length (min 8 chars)
- Added email and password validation utilities to lib/validation.ts
- Removed PrismaAdapter from auth.ts (using JWT sessions only)
- Added vitest testing framework and configuration
- Created comprehensive unit tests for registration functionality
- All tests passing (6/6)

Requirements covered: 1.1, 1.2, 1.3, 1.4, 1.5

package.json

@@ -7,6 +7,8 @@
     "build": "next build",
     "start": "next start",
     "lint": "next lint",
+    "test": "vitest --run",
+    "test:watch": "vitest",
     "db:push": "prisma db push",
     "db:studio": "prisma studio",
     "icons:generate": "node scripts/generate-icons.js",
@@ -29,12 +31,14 @@
     "@types/node": "^20",
     "@types/react": "^18",
     "@types/react-dom": "^18",
+    "@vitest/ui": "^4.0.14",
     "autoprefixer": "^10.0.1",
     "eslint": "^8",
     "eslint-config-next": "14.0.4",
     "postcss": "^8",
     "prisma": "^5.7.1",
     "tailwindcss": "^3.3.0",
-    "typescript": "^5"
+    "typescript": "^5",
+    "vitest": "^4.0.14"
   }
 }