fix: in-app browser download opens file inline

- Detect in-app WebViews (WeChat / TokenPocket / imToken / Telegram / iOS WKWebView, etc.) and show a guide modal asking the user to open the link in their system browser, with a copy-link action.
- For normal browsers, fetch the attachment as a Blob and trigger download from a same-origin object URL so the file always lands in the user's Downloads folder with the original filename, even when the browser would otherwise inline-preview the response.
- Fall back to the anchor download for files larger than 50MB (avoid loading them entirely into memory) or when fetch fails.
- Pass `sizeBytes` from known call sites so the threshold actually applies.
- Add localized strings for the guide modal in all 7 locales.

See .unipi/docs/debug/2026-06-05-in-app-browser-download-debug.md.

src/locales/zh-CN.ts

@@ -245,4 +245,16 @@ export const zhDict: Dict = {
   featureUnavailableDesc: "该功能暂未开放。",
   confirm: "知道了",
   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。",
+  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」。",
+  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。",
+  inAppDownloadCopied: "链接已复制",
+  inAppDownloadCopyFail: "复制失败，请手动复制",
 };