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/id.ts

@@ -267,4 +267,16 @@ export const idDict: Dict = {
   featureUnavailableDesc: "Fitur ini belum tersedia.",
   confirm: "Mengerti",
   backToHome: "Kembali ke Beranda",
+  inAppDownloadTitle: "Silakan buka di peramban sistem untuk mengunduh",
+  inAppDownloadIntro:
+    "Peramban dalam aplikasi saat ini tidak dapat mengunduh berkas. Buka halaman ini di peramban sistem (Safari, Chrome, dll.) lalu ketuk unduh lagi.",
+  inAppDownloadIntroNamed:
+    "{browser} tidak dapat mengunduh berkas secara langsung. Buka halaman ini di peramban sistem (Safari, Chrome, dll.) lalu ketuk unduh lagi.",
+  inAppDownloadStepCopy: "Ketuk “Salin tautan” di bawah.",
+  inAppDownloadStepOpen:
+    "Buka menu di kanan atas, pilih “Buka di peramban”, tempelkan tautan jika diperlukan.",
+  inAppDownloadStepDownload:
+    "Di peramban sistem, ketuk lagi tombol unduh dan berkas akan tersimpan.",
+  inAppDownloadCopied: "Tautan disalin",
+  inAppDownloadCopyFail: "Tidak dapat menyalin, silakan salin secara manual",
 };