[WIP] Fix collaborator role change issue

[Copilot]

• Explore codebase to understand existing admin/user role system
• Add is_admin field to UserDetailResponse schema
• Add UserRoleUpdateRequest schema to backend/app/schemas/admin.py
• Add update_user_admin_status CRUD function to backend/app/crud/user.py
• Add PATCH /admin/users/{user_id}/role endpoint to backend/app/api/v1/admin.py
• Add is_admin to User interface in frontend admin service
• Add changeUserRole method to adminService.ts
• Add role change UI (promote/demote to Admin) to admin users page

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