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/main.scss

@@ -0,0 +1,30 @@
+/**
+ * Entity Selector Styles
+ * @package prestashop-entity-selector
+ * @version 2.0.0
+ *
+ * Compiles to: assets/css/admin/entity-selector.css
+ */
+
+// Foundation
+@use 'variables' as *;
+@use 'mixins' as *;
+
+// Layouts
+@use 'layouts/form-integration';
+@use 'layouts/responsive';
+
+// Components
+@use 'components/entity-selector';
+@use 'components/dropdown';
+@use 'components/chips';
+@use 'components/groups';
+@use 'components/value-picker';
+@use 'components/modal';
+@use 'components/list-preview';
+@use 'components/schedule';
+@use 'components/tips';
+@use 'components/condition-trait';
+@use 'components/combinations';
+@use 'components/method-dropdown';
+@use 'components/tooltip';