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/components/LatestUpdateCard.tsx

@@ -38,7 +38,9 @@ function LatestActions({
     if (!attachment || isDownloading) return;
     setIsDownloading(true);
     try {
-      await downloadAttachment(post.id, attachment.id, attachment.filename);
+      await downloadAttachment(post.id, attachment.id, attachment.filename, {
+        sizeBytes: attachment.sizeBytes,
+      });
       const mediaKind = mediaSaveKindFromAttachment(attachment);
       if (mediaKind) showSaveToAlbumGuide(mediaKind);
     } catch {