Show desktop tool screenshots without digging through tool output #440

Found in the desktop tools study (#427), scenario D1 on Canary 0.0.46+dd15840. The earlier study's scenario 7 (F7.2) found the same thing.

Observed: the agent screenshotted TextEdit with a desktop tool and described it. To see the image, the tester had to open three nested disclosures and scroll past raw JSON, which took about 15 s. Confusion 2/3. Clicking the image opens it large, labeled "Image returned by desktop_click". The agent's final reply has neither the image nor a link to it.

Expected: an image a desktop tool returns is visible in the timeline at the tool call's top level, as a thumbnail, without expanding the raw result. The JSON can stay folded.

Part of #427.