Artifact Viewer close icon can disappear after auto-opening in a background chat

### Before filing

- [x] I searched [open and closed issues](https://github.com/block/berd/issues?q=is%3Aissue) for duplicates.
- [x] I reproduced this on the [latest release](https://github.com/block/berd/releases).
- [x] This is one bug, not several bundled together.

### Closest existing issue

None found. I searched open and closed issues for artifact/preview/code viewer, close button/icon, invisible/hidden/disappearing, clipping, and background-chat auto-open behavior.

### What's broken

After the Artifact Viewer appears automatically in a chat while that chat is in the background, returning to the chat can show the viewer without its top-right close **X**. The close button's hit target is still present and clickable, but the X icon is visually hidden behind or clipped by the viewer.

### Steps to reproduce

1. Launch Berd.
2. Open a chat with prior history using `gpt-5.6-sol` through the Goose harness.
3. Start an agent turn that creates a document which causes the Artifact Viewer to auto-open.
4. Navigate to another chat while that turn is running.
5. After the Artifact Viewer has apparently auto-opened in the background chat, navigate back to that chat.
6. Inspect the top-right corner of the Artifact Viewer.

This is intermittent; the sequence above describes the observed trigger rather than a guaranteed reproduction on every attempt.

### What you expected to happen

The Artifact Viewer header should always display its close X in the top-right corner, including when the viewer auto-opened while its chat was in the background.

### What actually happened

The X icon sometimes was not visible and appeared to be behind or clipped by the viewer. Clicking the location where the X should have been still closed the viewer, so the button remained interactive even though its icon was not painted visibly.

### How often does it happen?

Intermittently — happens often but not always

### Berd version

0.6.2

### Operating system

macOS (Apple Silicon), macOS 26.5.2 (build 25F84)

### Model and provider

`gpt-5.6-sol` through the Goose harness; underlying provider unknown. The symptom appears to be in the Artifact Viewer UI rather than model-specific behavior.

### Relevant log output

```text
no relevant log output
```

### Screenshots, recordings, or other context

No screenshot or recording was captured while the intermittent state was visible. The important distinguishing detail is that the close button remains clickable at its normal position; only the X icon disappears visually.