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/components/_method-dropdown.scss

@@ -82,14 +82,25 @@
 
     // Hidden select (for form submission)
     .method-select-hidden {
-        position: absolute;
-        opacity: 0;
-        pointer-events: none;
-        width: 0;
-        height: 0;
+        position: absolute !important;
+        opacity: 0 !important;
+        pointer-events: none !important;
+        width: 0 !important;
+        height: 0 !important;
+        overflow: hidden !important;
     }
 }
 
+// Global fallback for hidden method selects
+.method-select-hidden {
+    position: absolute !important;
+    opacity: 0 !important;
+    pointer-events: none !important;
+    width: 0 !important;
+    height: 0 !important;
+    overflow: hidden !important;
+}
+
 // =============================================================================
 // Method Dropdown Menu (appended to body, outside trait wrappers)
 // =============================================================================