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/_mixins.scss

@@ -196,6 +196,12 @@
         box-shadow: 0 2px 8px rgba($bg, 0.4);
     }
 
+    // Focus state - maintain styled appearance
+    &:focus {
+        outline: none;
+        box-shadow: 0 0 0 2px rgba($bg, 0.3), 0 2px 8px rgba($bg, 0.4);
+    }
+
     // Loading state - spinner icon replaces eye
     &.loading {
         cursor: wait;