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/_value-picker.scss

@@ -42,10 +42,16 @@
         }
     }
 
+    // Separation between chips and search box
+    .chips-wrapper + .entity-search-box {
+        margin-top: $es-spacing-md;
+    }
+
     .entity-search-icon {
         color: $es-text-muted;
         font-size: 14px;
         flex-shrink: 0;
+        margin-left: $es-spacing-xs;
     }
 
     // Override parent form's max-width on search input
@@ -85,6 +91,31 @@
         }
     }
 
+    // Browse tree button (for categories)
+    .btn-browse-tree {
+        @include button-reset;
+        display: flex;
+        align-items: center;
+        justify-content: center;
+        width: 32px;
+        height: 32px;
+        margin-left: auto;
+        color: $es-primary;
+        background: $es-primary-light;
+        border-radius: $es-radius-sm;
+        flex-shrink: 0;
+        transition: all $es-transition-fast;
+
+        &:hover {
+            background: $es-primary;
+            color: $es-white;
+        }
+
+        i {
+            font-size: 14px;
+        }
+    }
+
     // Numeric range box
     .numeric-range-box,
     .multi-range-input-row {