[Codex App][UX] Opening generated images should preserve the right sidebar width

Feature request

When Codex generates an image and I click it to preview, the right-side preview pane automatically expands to a much larger width (sometimes effectively taking over the workspace).

Current behavior

1. Resize the right sidebar / preview pane to a preferred width.
2. Generate an image in Codex.
3. Click the generated image.
4. The image preview opens, but Codex also changes the sidebar width and expands the pane.

This interrupts the existing workspace layout and repeatedly forces the user to resize the pane back.

Expected behavior

Opening an image preview should preserve the user's current right-sidebar width.

The image should scale to fit inside the existing pane (for example, equivalent to max-width: 100%; max-height: 100%; object-fit: contain) rather than resizing the pane to fit the image.

If a larger preview is useful, it should be an explicit action such as Expand preview / Full screen, not the default behavior.

Why this matters

Image-generation workflows involve opening many generated images in succession. Automatically expanding the preview pane each time causes unnecessary layout shifts and makes it harder to keep the conversation/code context visible.

Suggested behavior

• Persist the user-adjusted sidebar width.
• Open generated-image previews inside that width.
• Scale the image to fit the pane.
• Provide an explicit control to enlarge/full-screen the preview.
• Do not overwrite the saved pane width when entering or leaving image preview mode.

No files or tests are named. Start by locating the Codex App implementation for generated-image previews and the right sidebar width state, then trace what happens when an image is opened. Done means opening and closing a preview preserves the user's pane width and the image fits within that pane without an automatic resize.