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/_mixins.scss

@@ -3,6 +3,7 @@
  * Reusable patterns - prefer Bootstrap utilities in HTML where possible
  */
 
+@use "sass:color";
 @use 'variables' as *;
 
 // =============================================================================
@@ -231,7 +232,7 @@
 
     // Popover open state
     &.popover-open {
-        background: darken($bg, 10%);
+        background: color.adjust($bg, $lightness: -10%);
         box-shadow: 0 2px 8px rgba($bg, 0.4);
     }