Add complete CRUD interface for role management under /admin/roles.
- Index page with table showing name, description, permission_set_name, is_system_role
- Show page for role details
- Form component for create/edit with permission_set_name dropdown
- System role badge and disabled delete button
- Flash messages for success/error
- Authorization checks using MvWeb.Authorization helpers
- Comprehensive test coverage (22 tests)
Routes added under /admin scope. All LiveViews load user role
for authorization checks. Form uses custom dropdown for permission sets.
