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>

assets/js/admin/schedule-conditions.min.js

@@ -0,0 +1,5 @@
+// Placeholder - _core.js
+
+// Placeholder - _timeline.js
+
+//# sourceMappingURL=schedule-conditions.min.js.map

assets/js/admin/schedule-conditions.min.js.map

@@ -0,0 +1 @@
+{"version":3,"sources":[],"names":[],"mappings":"","file":"schedule-conditions.min.js","sourcesContent":[]}