Add delete user and set as admin button

[KeepingRunning]

Description

• Add backend API endpoints for user management:
  • DELETE /api/v1/user/{id} - Delete user by ID
  • PUT /api/v1/user/{id}/admin - Set user as admin
  • GET /api/v1/user/{id} - Get user by ID
• Enhance UserManagement UI with action buttons:
  • Add delete user button (hidden for current user)
  • Add set admin role button (for non-admin users)
  • Replace dropdown menu with direct action buttons
• Add comprehensive error handling and user feedback
• Include i18n support for new user management features
• Prevent users from deleting their own accounts
• Improve role display (show 'user' instead of 'Unknown')"

Related Issue

Close of #16