[6.x] Dashboard widget preferences

[jasonvarga]

Summary

• Adds per-user dashboard widget preferences, allowing widgets to be added, removed, reordered, and configured via the Control Panel
• Widget configuration is persisted to user preferences via a new DashboardWidgetsController
• Widget edit mode uses an overlay with a drag handle, width selector, configure, and remove actions
• Widget picker uses a dropdown instead of a full-width stack panel
• Dashboard components (WidgetConfigStack, WidgetPicker, WidgetEditOverlay) and Dashboard.vue are all written using the Composition API (<script setup>)
• Removed description() method from widget classes as it is unused in the UI

Test plan

• Enable edit mode on the dashboard and add widgets from the dropdown picker
• Configure a widget and confirm settings are saved and applied
• Reorder widgets via drag and drop
• Remove a widget
• Change widget width using the width selector in the edit overlay
• Save and confirm preferences persist after page reload
• Cancel editing and confirm no changes are applied