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.

env.example

@@ -14,6 +14,10 @@ JIRA_BASE_URL=""                    # https://votre-domaine.atlassian.net
 JIRA_EMAIL=""                       # votre.email@domaine.com
 JIRA_API_TOKEN=""                   # Token API Jira
 
+# NextAuth (requis)
+NEXTAUTH_URL="http://localhost:3000"  # URL de votre application
+NEXTAUTH_SECRET="your-secret-key-here" # Clé secrète pour signer les tokens
+
 # Debug (optionnel)
 VERBOSE_LOGGING="false"             # Logs détaillés en développement
 NODE_ENV="development"              # development | production