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>

assets/admin/src/components/form/repeater-rows.js

@@ -41,10 +41,21 @@ function rowDuration(row, cfg) {
   return String(raw)
 }
 
+/** 未配置 titleKey 时，取首个值为非空字符串/数字的字段作为标题。 */
+function fallbackTitleKey(row, fields) {
+  if (row.title != null && String(row.title).trim() !== '') return 'title'
+  for (const field of (Array.isArray(fields) ? fields : [])) {
+    if (field.ui_only) continue
+    const value = row[field.key]
+    if ((typeof value === 'string' && value.trim() !== '') || typeof value === 'number') return field.key
+  }
+  return 'title'
+}
+
 /** 收起态标题行上的一行摘要，取自 listPreview 配置。 */
-export function repeaterRowSummary(row, cfg = {}) {
+export function repeaterRowSummary(row, cfg = {}, fields = []) {
   if (!row) return ''
-  const title = String(row[cfg.titleKey || 'title'] ?? '').trim()
+  const title = String(row[cfg.titleKey || fallbackTitleKey(row, fields)] ?? '').trim()
   const prefix = String(cfg.prefix || '')
   if (title === '') return prefix
   const duration = rowDuration(row, cfg)