feat(admin): make all sortable/repeater cards collapsible, collapsed by default

- Add useCollapsibleItems: key-based open state, clickable/keyboard head, aria-expanded
- sortable items with sub-fields and every inline repeater/group row now collapse
- Repeater head summary falls back to the first filled field when no listPreview
- Drop per-schema collapsible/defaultCollapsed for repeater groups (JS + PHP)
- Align FloatBtnItemPanel head with the shared collapse attrs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/widget-system/vip-settings-structured-harness.php

@@ -267,8 +267,6 @@ namespace {
 						foreach ($tab['fields'] as $field) {
 							if (($field['type'] ?? '') === 'group') {
 								$out['groups'][$tab['key'] . '/' . $field['key']] = [
-									'collapsible' => $field['collapsible'] ?? null,
-									'defaultCollapsed' => $field['defaultCollapsed'] ?? null,
 									'listPreview' => $field['listPreview'] ?? null,
 								];
 							}
@@ -294,8 +292,6 @@ namespace {
 						continue;
 					}
 					$out['groups']['points-exchange/' . $field['key']] = [
-						'collapsible' => $field['collapsible'] ?? null,
-						'defaultCollapsed' => $field['defaultCollapsed'] ?? null,
 						'listPreview' => $field['listPreview'] ?? null,
 					];
 				}