Add a slide-out drawer UI to browse available widgets from the registry,
search by name/description, and add them to the dashboard. The picker
is accessible via "Add Widget" button in edit mode.
- WidgetPicker component with search, widget list, and one-click add
- Generates unique widget IDs and places new widgets below existing ones
- Integrated into dashboard page header (edit mode only)
Ref: #488
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
