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/en.ts

@@ -264,4 +264,16 @@ export const enDict: Dict = {
   featureUnavailableDesc: "This feature is not available yet.",
   confirm: "Got it",
   backToHome: "Back to Home",
+  inAppDownloadTitle: "Please open in your system browser to download",
+  inAppDownloadIntro:
+    "Your current in-app browser cannot download files. Open the page in your system browser (Safari, Chrome, etc.) and tap download again.",
+  inAppDownloadIntroNamed:
+    "{browser} cannot download files directly. Open the page in your system browser (Safari, Chrome, etc.) and tap download again.",
+  inAppDownloadStepCopy: "Tap “Copy link” below.",
+  inAppDownloadStepOpen:
+    "Open the menu (top-right), choose “Open in browser”, and paste the link if needed.",
+  inAppDownloadStepDownload:
+    "Tap the download button on the page — the file will save to your downloads.",
+  inAppDownloadCopied: "Link copied",
+  inAppDownloadCopyFail: "Could not copy the link, please copy it manually",
 };