Refactor preview popover and fix dropdown header styling

- Refactor preview popover into reusable component with filter input and load more
- Fix sort controls: connect sort-field-select to btn-sort-dir (shared border)
- Fix refine controls: connect btn-refine-negate to refine-input
- Add flex-shrink: 0 and min-width to prevent button shrinking
- Fix grid view items: border on all 4 sides, no padding
- Add EXCEPT separator styling for selection groups
- Various button width and height fixes for consistency

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

sources/scss/_mixins.scss

@@ -21,6 +21,12 @@
     justify-content: space-between;
 }
 
+@mixin grid-columns($cols) {
+    display: grid;
+    grid-template-columns: repeat($cols, 1fr);
+    gap: $es-spacing-sm;
+}
+
 // =============================================================================
 // Text
 // =============================================================================