Previously, handle_view_image called td_client.download_file() synchronously
while process_pending_chat_init already had a background synchronous=true
download in flight for the same file. TDLib returned is_downloading_completed=false
causing the view to fail on first press.
Fix: replace the blocking download in NotDownloaded/Downloading branches with
a pending_image_open intent flag. The main loop opens the modal automatically
when the background download result arrives via photo_download_rx. If no
background channel exists, a new one is started via direct tdlib_rs call.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
