refactor(admin): unify sortable/draggable/repeater list UI

- Add shared sortable-list.css, SORTABLE_LIST_ATTRS and AksSortHandle
- Migrate schema sortable, selected order, inline/modal repeater, sorter,
  floating buttons, footer tabbar and ticket prereply lists to the shared design
- Split AksField.vue (1885 -> 969 lines) into focused components/helpers
- Remove dead AksArrayTable/AksRepeaterField and move-up/down helpers
- Replace hardcoded colors and --n-* vars with --aksrt-color-* tokens

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

assets/admin/src/style.css

@@ -341,6 +341,16 @@
   max-width: 520px;
 }
 
+/* 弹层底部操作栏；弹层 teleport 到 #aksrt-admin-app 之外，token 带回退值。 */
+.aksrt-modal-footer {
+  display: flex;
+  justify-content: flex-end;
+  gap: 8px;
+  margin-top: 16px;
+  padding-top: 12px;
+  border-top: 1px solid var(--aksrt-color-border, #E2E8E3);
+}
+
 @media (max-width: 960px) {
   #aksrt-admin-app .aksrt-form-renderer__save-bar {
     right: 0;