Administration: Give the admin menu a stable view transition name.

[dhrupo]

Fixes the admin menu flicker/crossfade during view transitions by giving the sidebar containers stable view-transition-name values.

Trac ticket: https://core.trac.wordpress.org/ticket/65032

Testing

• Run trunk in Chrome.
• Log in to wp-admin.
• Navigate between screens with similar sidebars, for example Dashboard -> Posts -> Pages.
• Confirm the admin menu is no longer treated as part of the root transition snapshot.

Notes

• Verified locally that #adminmenuwrap and #adminmenuback receive separate view-transition groups during navigation in Chrome via Playwright/runtime inspection.

[github-actions]

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Props dhrupo, huzaifaalmesbah.

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[github-actions]

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[huzaifaalmesbah]

After applying this patch, I noticed an issue. you can check it in the video.

after.apply.patch.mp4

[dhrupo]

Confirmed this is a valid issue. #adminmenuback is a fixed background layer, and giving it its own view-transition-name can produce a bad transition snapshot. I pushed c06aea7 to narrow the patch so only #adminmenuwrap participates, while leaving the menu item transition names in place.