feat: add authentication support and user model

- Updated `env.example` to include NextAuth configuration for authentication.
- Added `next-auth` dependency to manage user sessions.
- Introduced `User` model in Prisma schema with fields for user details and password hashing.
- Integrated `AuthProvider` in layout for session management across the app.
- Enhanced `Header` component with `AuthButton` for user authentication controls.

prisma/schema.prisma

@@ -7,6 +7,23 @@ datasource db {
   url      = env("DATABASE_URL")
 }
 
+model User {
+  id          String   @id @default(cuid())
+  email       String   @unique
+  name        String?
+  firstName   String?
+  lastName    String?
+  avatar      String?  // URL de l'avatar
+  role        String   @default("user") // user, admin, etc.
+  isActive    Boolean  @default(true)
+  lastLoginAt DateTime?
+  password    String   // Hashé avec bcrypt
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
+
+  @@map("users")
+}
+
 model Task {
   id               String          @id @default(cuid())
   title            String