fix: improve SearchableSelect with label resolution for pre-selected items

- Fetch post titles for pre-selected IDs on mount via include[] param
- Cache labels from search results to avoid re-fetching
- Filter already-selected items from dropdown results
- Add loading spinner and no-results message
- Update PHP autocomplete handler to support include[] param
- Fix CSS class conflict warning

build/admin.asset.php

@@ -1 +1 @@
-<?php return array('dependencies' => array('react', 'react-dom', 'wp-components', 'wp-element', 'wp-i18n', 'wp-icons/build/arrow-left', 'wp-icons/build/bell', 'wp-icons/build/message', 'wp-icons/build/trash', 'wp-primitives'), 'version' => 'f5d52a62f3d00d92014a');
+<?php return array('dependencies' => array('react', 'react-dom', 'wp-components', 'wp-element', 'wp-i18n', 'wp-icons/build/arrow-left', 'wp-icons/build/bell', 'wp-icons/build/message', 'wp-icons/build/trash', 'wp-primitives'), 'version' => '09bb3ab758733bb11e68');