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/lib/postFile.ts

@@ -0,0 +1,58 @@
+/** 帖子附件客户端校验（与后端 setting / fileallow 对齐；安全边界仍在 Go） */
+
+export const FILE_MAX_MB_DEFAULT = 20;
+export const FILE_MAX_COUNT_DEFAULT = 10;
+
+export function normalizeExt(nameOrExt: string): string {
+  const base = nameOrExt.includes("/")
+    ? nameOrExt.split(/[/\\]/).pop() || ""
+    : nameOrExt;
+  const i = base.lastIndexOf(".");
+  const ext = (i >= 0 ? base.slice(i + 1) : base).trim().toLowerCase();
+  return ext.replace(/[^a-z0-9]/g, "").slice(0, 16);
+}
+
+export function buildAcceptAttr(exts: string[], limit: boolean): string | undefined {
+  if (!limit) return undefined;
+  if (!exts.length) return undefined;
+  return exts.map((e) => `.${e}`).join(",");
+}
+
+export function isAllowedPostFileName(
+  name: string,
+  opts: { limit: boolean; exts: string[] }
+): boolean {
+  if (!opts.limit) return true;
+  const ext = normalizeExt(name);
+  if (!ext) return false;
+  return opts.exts.includes(ext);
+}
+
+export function formatMB(n: number): string {
+  return `${n}MB`;
+}
+
+export type PostFileRejectReason = "size" | "type" | "count" | null;
+
+export function rejectPostFile(
+  file: File,
+  opts: {
+    limit: boolean;
+    exts: string[];
+    maxMB: number;
+    maxCount: number;
+    currentCount: number;
+  }
+): { reason: PostFileRejectReason; message?: string } {
+  if (opts.currentCount >= opts.maxCount) {
+    return { reason: "count", message: `最多 ${opts.maxCount} 个附件` };
+  }
+  const maxBytes = opts.maxMB * 1024 * 1024;
+  if (file.size > maxBytes) {
+    return { reason: "size", message: `附件不能超过 ${opts.maxMB}MB` };
+  }
+  if (!isAllowedPostFileName(file.name, opts)) {
+    return { reason: "type", message: "不支持该附件格式" };
+  }
+  return { reason: null };
+}