Add hierarchical tree view for category selection

Features:
- Tree view mode for categories with expand/collapse
- Product count badges with clickable preview popover
- Select parent with all children button
- Client-side tree filtering (refine search)
- Keyboard shortcuts: Ctrl+A (select all), Ctrl+D (clear)
- View mode switching between tree/list/columns
- Tree view as default for categories, respects user preference

Backend:
- Add previewCategoryProducts and previewCategoryPages AJAX handlers
- Support pagination and filtering in category previews

Styling:
- Consistent count-badge styling across tree and other views
- Loading and popover-open states for count badges

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

sources/scss/main.scss

@@ -28,3 +28,4 @@
 @use 'components/combinations';
 @use 'components/method-dropdown';
 @use 'components/tooltip';
+@use 'components/tree';