Add schedule preview dropdown styles and various improvements

- Add .schedule-preview-dropdown and .schedule-preview-item CSS classes
- Add .btn-schedule-preview badge styling
- Add preview functionality for entity list views
- Improve modal and dropdown styling
- Various JS and SCSS enhancements

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

sources/scss/components/_tree.scss

@@ -3,6 +3,7 @@
  * Hierarchical tree view for category selection inside dropdown
  */
 
+@use "sass:color";
 @use '../variables' as *;
 @use '../mixins' as *;
 
@@ -231,7 +232,7 @@
             }
 
             &.popover-open {
-                background: darken($es-primary, 10%);
+                background: color.adjust($es-primary, $lightness: -10%);
             }
         }
     }