feat(ui): Complete Phase 8 UI integration

- Add view preset buttons to left panel (Top, Bottom, Front, Back, Left, Right, Isometric)
- Add user commands panel with category organization
- Implement script editor dialog with code editing
- Add menu items for script editor and user commands toggle
- Integrate Phase 8 features with existing UI

UI Components:
- View Presets group with 7 preset buttons
- User Commands panel with category-based command display
- Script Editor modal with name, code, and toolbar options
- Menu bar extensions for Tools and View menus

New Methods:
- RCandleApp::apply_view_preset() - Apply camera view preset
- RCandleApp::execute_user_command() - Execute user command
- RCandleApp::show_script_editor_window() - Render script editor
- Renderer::calculate_bounds() - Calculate toolpath bounds
- Renderer::apply_view_preset() - Apply view to camera

Code Quality:
- Resolved all borrow checker issues with action queue pattern
- Clean compilation with only warnings
- Proper nalgebra Point3 type conversions

Status: UI integration complete, backend wiring pending

Author: thawkins