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/floating/FloatBtnItemPanel.vue

@@ -1,6 +1,6 @@
 <template>
   <div class="float-btn-panel">
-    <div class="aksrt-sortable__head is-clickable" @click="expanded = !expanded">
+    <div class="aksrt-sortable__head" v-bind="headAttrs">
       <AksSortHandle @click.stop />
       <div class="aksrt-sortable__main">
         <div class="aksrt-sortable__title-row">
@@ -67,6 +67,7 @@ import { computed, ref } from 'vue'
 import FaIconPicker from '@/components/form/FaIconPicker.vue'
 import AksMediaField from '@/components/form/AksMediaField.vue'
 import AksSortHandle from '@/components/form/AksSortHandle.vue'
+import { collapseHeadAttrs } from '@/components/form/useCollapsibleItems.js'
 import FloatBtnCommonFields from './FloatBtnCommonFields.vue'
 import FloatMoreButtonsEditor from './FloatMoreButtonsEditor.vue'
 import { FLOAT_BTN_META } from './floating-defaults.js'
@@ -77,6 +78,7 @@ const props = defineProps({
 const emit = defineEmits(['update:modelValue'])
 
 const expanded = ref(false)
+const headAttrs = computed(() => collapseHeadAttrs(expanded.value, () => { expanded.value = !expanded.value }))
 const meta = computed(() => FLOAT_BTN_META[props.modelValue.type] || { title: props.modelValue.type, panel: '' })
 
 const addIcon = computed({