Backup: render the modernized dashboard inside a Page chassis

[keoshi]

Refs #48582

Proposed changes

PR 2 of eight in the Backup modernization umbrella. Replaces PR 1's <h1>Backup</h1> placeholder with Page from @wordpress/admin-ui (title "VaultPress Backup", subtitle, empty body) so the modernized surface picks up the standard wp-admin chrome. Suppresses admin_notices and all_admin_notices on the modernized page only, mirroring how Jetpack Forms keeps its dashboard from reflowing on top of JITMs and other plugin notices.

No data layer, no gates, no router yet — those land in PR 2.5 (update/backup-modernization-data-layer). The modernization filter (rsm_jetpack_ui_modernization_backup) still defaults to false, so today's Backup admin page renders unchanged for every site.

Mirrors the cadence of #48574 (Newsletter page-shell).

Testing instructions

1. Filter OFF — legacy Backup admin page is unchanged. Visit Jetpack → VaultPress Backup. The page renders today's standalone UI exactly as it did before this PR. Any admin_notices / all_admin_notices registered by core or other plugins still render — this PR doesn't touch them.

2. Filter ON — modernized chassis renders. Add to a mu-plugin:

   add_filter( 'rsm_jetpack_ui_modernization_backup', '__return_true' );

   Reload Jetpack → VaultPress Backup. The page now renders the wp-admin Page chassis: a header with the VaultPress Backup title and the Save changes and restore quickly with one-click recovery. subtitle, followed by an empty body. Any admin_notices registered above (e.g. via add_action( 'admin_notices', fn() => print '<div class=\"notice notice-warning\"><p>Test</p></div>' );) are suppressed.

3. Toggle the filter back off. Reload — legacy Backup returns immediately, no cache clears.

Does this pull request change what data or activity we track or use?

No. No new endpoints, payloads, identifiers, Tracks events, or persistent options. The new Page shell renders an empty body — there is no data flow yet.

Out of scope

• No data layer, no gates, no router, no REST bridges (PR 2.5).
• No Overview, file browser, Download, or Restore screens (PRs 3-6).

References

• Umbrella: Backup: modernize the wp-admin overview into a unified product (seven-PR sequence) #48582
• Working reference (kept open until PR 7 lands): Backup: New overview UI #48236
• Predecessor: Backup: add empty wp-build dashboard scaffold behind feature flag #48583 (PR 1 — scaffold)
• Cadence template: Newsletter: scaffold the wp-build dashboard chassis behind the flag #48574 (Newsletter page-shell)

🤖 Generated with Claude Code

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack or WordPress.com Site Helper), and enable the update/backup-modernization-page-shell branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack update/backup-modernization-page-shell

bin/jetpack-downloader test jetpack-mu-wpcom-plugin update/backup-modernization-page-shell

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• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
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[github-actions]

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/backup/src/class-jetpack-backup.php	53/407 (13.02%)	-0.10%	3 ❤️‍🩹

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