Preview mode is redundant with the browser pop-up and leaves two browser views open #22041

Problem

Preview mode doesn't add much over the existing browser pop-up, and it creates a confusing state:

1. It's redundant. The browser pop-up can already be resized larger, so preview mode doesn't give me anything I can't already get.
2. It duplicates the browser. Entering preview mode doesn't close or hide the existing browser view, so I end up with two browser views of the same page open at once.

Proposed change

• Merge preview mode into the browser pop-up: "preview" becomes an expanded or maximized state of the same view (see the double-click request in Feature request: Double-click the mini browser pop-up to expand it #22040), not a separate surface.
• Or, at minimum, entering preview mode should hide or replace the existing browser view so only one is ever visible.

Possibly related: #16901 (agent and browser panel showing different pages for the same tab).

Question for maintainers

Would this be welcome? Happy to help with a PR if the direction fits.