[Bug]: iPhone chat shows "Image unavailable" for host-path Markdown images outside the project #17668

What happened

An assistant message embeds a screenshot stored on the host with a Markdown image and an absolute path. In the iPhone app the image is replaced by "Image unavailable".

Two messages in the same thread, same PNG (1309×1410, 292 KB), both fail on the iPhone:

![Synara now](/tmp/synara-shot/synara-now.png)
![Synara now](/Users/<user>/.t3/userdata/browser-artifacts/synara-now.png)

The second path is the folder preview_snapshot with save=true writes to, which its tool description says is the way to show the user a screenshot. The file there was copied in by the agent, not written by preview_snapshot.

Expected

The image renders in the iPhone chat, as host-path embeds are meant to through the signed media-file URL (#9023).

Environment

Not verified

• iOS app version and iOS version.
• How the phone reaches the host (T3 Connect or local network).
• Whether the same two messages render on desktop.
• Whether an in-project relative path or an HTTPS image renders on the same phone.

Related

• Android host-path Markdown image is blank while desktop and HTTPS embeds work #10322: same shape on Android (blank tile), closed as fixed in Android 1.0.4.
• fix(mobile): iPhone chat pictures were empty gray boxes #9087: open PR for empty gray boxes on iPhone. This report shows the explicit "Image unavailable" state instead, so the request for the image is being refused or failing rather than never made.
• [Bug]: preview_snapshot returns raw Windows backslash path breaking chat image rendering (Image unavailable) #14839, [Bug]: Images and file links with Windows paths like C:\Users\me\.t3\... don't load #14261: "Image unavailable" caused by Windows backslash paths; these paths are POSIX.