Full-screen image viewer regression: cannot dismiss with Esc or backdrop click

After the recent change from the previous side-panel image preview to the new full-screen image viewer, opening and closing images has become unnecessarily cumbersome.

Currently:

• Esc does not close the image viewer.
• Clicking the backdrop / empty area outside the image does not close it.
• The only way to return to the conversation is to locate and click the dedicated close control.

In Codex, screenshots and images are often opened briefly for inspection and then dismissed immediately to continue working. This makes the new behavior especially disruptive in a frequent workflow.

The broader regression from the previous side-panel image preview to the new full-screen viewer is already being discussed in #38858.

Even if the full-screen viewer is retained, it should still behave like a standard modal/lightbox and support quick dismissal via:

• Esc
• Backdrop click
• The existing close button

Related: #38858

Start by locating the full-screen image viewer entry point and reproduce opening an image, pressing Esc, and clicking the backdrop. Compare the current behavior with the dismissal requirements in this issue and the related discussion in #38858. Done means Esc, backdrop click, and the existing close button all dismiss the viewer.