Revisions: Show design system focus ring on the code diff - #85368

What?

Part of #80138

The code diff in the revisions screen (Code editor → Revisions) now uses the design system's outline focus ring instead of a legacy box-shadow.

Why?

The ring was an inset box-shadow with outline: none, so keyboard users had no focus indicator in high contrast (forced colors) mode.

How?

• Replaced the box-shadow with outset-ring__focus().
• Set a negative outline-offset so the ring stays inside the region.

Testing Instructions

1. Open a post that has a few revisions.
2. Switch to the Code editor (⋮ → Code editor).
3. In the post sidebar, click "Revisions" to open the revisions screen.
4. Press Tab until the code diff has focus. It should show a ring along its inside edge.
5. Click the code diff with the mouse. There should be no ring.
6. In DevTools → Rendering, emulate forced-colors: active and tick "Emulate a focused page", then repeat step 4. The ring should be visible. On trunk there's no indicator.

Testing Instructions for Keyboard

1. Open the revisions screen with the Code editor active.
2. Press Tab until focus reaches the code diff region. It should show the focus ring.
3. Use the arrow keys to scroll the diff. The ring should stay visible.

Screenshots or screencast

	Before	After
Normal
Windows High Contrast Mode

Use of AI Tools

This PR was developed with the assistance of Claude Code.