feat(attach): 附件可配置限额、流式落盘与分阶段上传

管理端可配类型/体积；上传改流式避免整文件进内存；发帖先选再传并拦截未上传提交；提高 Next proxy 体积上限。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/components/admin/index.ts

@@ -14,3 +14,5 @@ export { default as AdminIconButton } from "./AdminIconButton";
 export { default as AdminStatusChip, contentStatusChip } from "./AdminStatusChip";
 export type { AdminStatusTone } from "./AdminStatusChip";
 export { default as AdminShell } from "./AdminShell";
+export { default as AdminExtChipEditor } from "./AdminExtChipEditor";
+export { ATTACHMENT_EXT_GROUPS } from "./AdminExtChipEditor";