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/useCollapsibleItems.js

@@ -0,0 +1,46 @@
+/**
+ * 可排序卡片列表的折叠状态：所有带展开区的卡片默认收起，点击行头展开/收起。
+ *
+ * 状态按行的稳定 key 记录（而不是下标），拖拽换位后每张卡片保持自己的开合状态。
+ * 行头属性统一由 collapseHeadAttrs 生成：可点击、可键盘操作（Enter / Space）、
+ * 带 aria-expanded，样式见 styles/sortable-list.css。
+ */
+import { ref } from 'vue'
+
+export function collapseHeadAttrs(open, toggle) {
+  return {
+    class: 'is-clickable',
+    role: 'button',
+    tabindex: 0,
+    'aria-expanded': open ? 'true' : 'false',
+    onClick: toggle,
+    onKeydown: (event) => {
+      // 只响应行头本身的按键，行内按钮 / 输入框的 Enter、空格不触发折叠。
+      if (event.target !== event.currentTarget) return
+      if (event.key !== 'Enter' && event.key !== ' ') return
+      event.preventDefault()
+      toggle()
+    },
+  }
+}
+
+export function useCollapsibleItems() {
+  const openKeys = ref(new Set())
+
+  function isOpen(key) {
+    return openKeys.value.has(key)
+  }
+
+  function toggle(key) {
+    const next = new Set(openKeys.value)
+    if (next.has(key)) next.delete(key)
+    else next.add(key)
+    openKeys.value = next
+  }
+
+  function headAttrs(key) {
+    return collapseHeadAttrs(isOpen(key), () => toggle(key))
+  }
+
+  return { isOpen, toggle, headAttrs }
+}