fix: use absolute URL in in-app download guide

Production builds the frontend with VITE_API_URL="" so attachmentDownloadUrl() returns a relative path like /apnew/api/.../download. Pasting that into Safari from another origin fails. Convert the URL to an absolute one (window.location.origin + path) before showing it in the in-app browser guide, and update the modal text in all 7 locales to make clear the copied link is the direct file download URL that, when opened in the system browser, triggers the download automatically.

src/locales/zh-CN.ts

@@ -247,14 +247,13 @@ export const zhDict: Dict = {
   backToHome: "返回首页",
   inAppDownloadTitle: "请使用系统浏览器打开后下载",
   inAppDownloadIntro:
-    "当前内置浏览器无法直接下载文件，请在系统浏览器（Safari、Chrome 等）中打开页面后再点击下载。",
+    "当前内置浏览器无法下载文件。复制下方链接，到系统浏览器打开即可直接下载。",
   inAppDownloadIntroNamed:
-    "{browser} 内置浏览器无法直接下载文件，请在系统浏览器（Safari、Chrome 等）中打开页面后再点击下载。",
-  inAppDownloadStepCopy: "点击下方「复制链接」。",
+    "{browser} 内置浏览器无法下载文件。复制下方链接，到系统浏览器打开即可直接下载。",
+  inAppDownloadStepCopy: "点击下方「复制链接」（这是文件的直接下载地址）。",
   inAppDownloadStepOpen:
-    "点右上角菜单，选择「在浏览器中打开」，如需要请把刚才复制的链接粘贴进地址栏。",
-  inAppDownloadStepDownload:
-    "在系统浏览器中再次点击下载按钮，文件会保存到下载文件夹。",
+    "打开系统浏览器（Safari、Chrome 等），把链接粘贴到地址栏。",
+  inAppDownloadStepDownload: "文件会自动开始下载到下载文件夹。",
   inAppDownloadCopied: "链接已复制",
   inAppDownloadCopyFail: "复制失败，请手动复制",
 };