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>

src/ScheduleConditions.php

@@ -789,7 +789,7 @@ trait ScheduleConditions
         $jsConfigJson = htmlspecialchars(json_encode($jsConfig), ENT_QUOTES, 'UTF-8');
 
         $html = '<div class="target-conditions-trait holiday-countries-target"';
-        $html .= ' data-target-conditions-id="holiday-countries-target"';
+        $html .= ' data-entity-selector-id="holiday-countries-target"';
         $html .= ' data-mode="multi"';
         $html .= ' data-config=\'' . $jsConfigJson . '\'>';