#79 UI: Create User Modal

Create modal dialog for adding new users. Component: api/ui/templates/components/admin_user_modal/ Files: - admin_user_modal.html - admin_user_modal.css - admin_user_modal.js Features: - Modal overlay with form - Form fields: Email (required), Name (optional), Role (dropdown) - Email validation (format and domain if restricted) - Role dropdown with descriptions - Send invitation checkbox (default: true) - Custom welcome message textarea - Cancel and Create buttons - Loading state during submission - Error display for validation failures - Success feedback with user details - Close on escape key - Close on overlay click (with confirmation if dirty) Validation: - Email format validation - Check if email already exists (debounced API call) - Role selection required With ultra attention to details and correctness. Before closing: write comment summarizing implementation with screenshot.