feat(widgets): add BaseFormDialog and TabbedFormWidget registry pattern

- Add BaseManagedWindow/BaseFormDialog for singleton window management
  with WindowManager integration
- Add TabbedFormWidget registry pattern documentation explaining
  dynamic config discovery and reactive state management
- Fix scoped_border_mixin paintEvent for QDialog compatibility
  (safe super() call for multiple inheritance)
- Add scope color scheme propagation to nested GroupBox containers
  in widget_creation_config.py
- Refactor system_monitor to compact horizontal layout with fixed height
  (200px), minimal margins, and better graph arrangement
- Update ParameterFormManager to store full scope color scheme
  for proper nested container styling

This enables the declarative window system in OpenHCS main GUI.

Author: trissim

docs/architecture/index.rst

@@ -15,6 +15,7 @@ Core Form System
    parametric_widget_creation
    widget_protocol_system
    field_change_dispatcher
+   tabbed_form_registry_pattern
 
 Service Layer
 -------------