task-7: buildDownloadUrl()/resolveDownloadUrl() 下载 URL 统一构造（user_id 追加与去重、fresh 优先）

frontend-vue/src/shared/api/download.ts

@@ -0,0 +1,22 @@
+export interface DownloadableItem {
+  freshDownloadUrl?: string | null
+  downloadUrl?: string | null
+}
+
+export function buildDownloadUrl(path: string, userId?: number): string {
+  if (userId === undefined || userId === null || Number.isNaN(userId)) {
+    return path
+  }
+  if (/[?&]user_id=/.test(path)) {
+    return path
+  }
+  const separator = path.includes('?') ? '&' : '?'
+  return `${path}${separator}user_id=${userId}`
+}
+
+export function resolveDownloadUrl(item: DownloadableItem | null | undefined): string {
+  if (!item) {
+    return ''
+  }
+  return item.freshDownloadUrl || item.downloadUrl || ''
+}