Stop exposing the internal /apnew/api/.../download URL in the guide. The dialog no longer renders the file URL or copies it to the clipboard; instead the user copies the current page link (window.location.href) and opens it in their system browser, then taps download again — which uses the real fetch+blob path. Updated step copy and intro in all 7 locales to match the new flow.
- 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.
