Add per-widget configuration dialog (WidgetConfigDialog) with settings
infrastructure ready for future configSchema-based forms. Add layout
management controls: reset to default, edit/done toggle, and widget
gear icon for configuration. Wire onEditWidget through WidgetGrid to
BaseWidget.
Implements TW-WDG-004 and TW-WDG-005.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>