fix(chat): inline preview generated image outputs

T46 devapp evidence shows browser screenshot output gov-cn-homepage.png is persisted and downloadable, but the assistant message renders only the generic produced-file card. Render generated image outputs as the real inline image, preserve download, and open an enlarged preview on click. Scope: chat produced-file rendering only; browser runtime and document publishing are unchanged.

Start at the chat produced-file rendering path and inspect how the generic produced-file card handles the browser screenshot output gov-cn-homepage.png. Verify the rendered result in the chat UI, including download behavior and click-to-enlarge preview. Done means generated image outputs appear inline while remaining downloadable and previewable.

Written by the indexing model from the issue text.