Refactor from Tailwind CSS to SCSS/Bootstrap

Major changes:
- Replace Tailwind with SCSS using Bootstrap-compatible variables
- Add Gulp build system for SCSS and JS compilation
- Split JS into modular partials (_events, _filters, _preview, etc.)

CSS fixes:
- Fix preview popover visibility (remove conflicting modal.scss rule)
- Fix search input max-width override for parent form styles
- Add filter panel styling (toggle buttons, chips, values row)
- Add group-body padding with negative margins on modifiers
- Style filter-group-toggle with eye icon for preview

JS additions:
- Add showGroupPreviewPopover for group count badge clicks
- Add showItemsPopover for rendering preview popover
- Add renderPreviewItems for product list rendering
- Add eye icon to filter toggle button generation

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

sources/scss/layouts/_responsive.scss

@@ -0,0 +1,63 @@
+/**
+ * Responsive Styles
+ * Media query adjustments for different screen sizes
+ */
+
+@use '../variables' as *;
+
+// Tablet and below
+@media (max-width: 991px) {
+    .target-conditions-trait,
+    .entity-selector-trait {
+        .condition-trait-header {
+            flex-direction: column;
+            align-items: flex-start;
+            gap: $es-spacing-sm;
+        }
+
+        .trait-header-right {
+            width: 100%;
+            justify-content: flex-end;
+        }
+
+        .target-block-tabs {
+            flex-wrap: wrap;
+        }
+    }
+}
+
+// Mobile
+@media (max-width: 767px) {
+    .target-conditions-trait,
+    .entity-selector-trait {
+        .target-block-tab {
+            padding: $es-spacing-sm;
+            font-size: $es-font-size-xs;
+        }
+
+        .target-group-header {
+            flex-direction: column;
+            align-items: flex-start;
+        }
+
+        .target-search-dropdown {
+            width: 100% !important;
+            left: 0 !important;
+            right: 0 !important;
+        }
+
+        .dropdown-results-grid {
+            grid-template-columns: 1fr !important;
+        }
+    }
+}
+
+// High-resolution displays
+@media (min-width: 1600px) {
+    .target-conditions-trait,
+    .entity-selector-trait {
+        .dropdown-results-grid.view-grid-3 {
+            grid-template-columns: repeat(4, 1fr);
+        }
+    }
+}