Newsletter: modernize into a unified wp-admin product (six-PR sequence)

[keoshi]

Goal

Modernize Jetpack's Newsletter product into a single, unified wp-admin destination with two tabs:

1. Subscribers (default) — the DataViews-based subscribers UI shipped in #48386.
2. Settings — every setting from the existing projects/packages/newsletter/ settings screen, ported onto the new chassis.

After this work the standalone "Subscribers" wp-admin entry goes away. There is one menu item: Newsletter.

This umbrella supersedes the prototype umbrella (#48416) and PR (#48420). The prototype is the working reference; this umbrella is the shipping plan.

Approach

Six small, self-contained PRs. Each builds on the previous one. Everything from PR 1 onward is gated behind a feature filter (rsm_jetpack_ui_modernization_newsletter) — the legacy Newsletter Settings page renders unchanged unless a site explicitly opts in. The final PR flips the default and retires the legacy surface.

This mirrors the cadence of the VideoPress wp-build scaffold (#48494).

PRs

• PR 1 — Empty wp-build dashboard scaffold behind feature flag. #48532 — wp-build alongside webpack; modernization filter + helpers; branched menu render; legacy enqueue short-circuit; placeholder route renders <h1>Newsletter</h1>. No tabs, no Subscribers, no Settings move.
• PR 2 — Page + @wordpress/route shell behind flag. #48574 — Replace the placeholder with Page from @wordpress/admin-ui + QueryClientProvider, lift analytics init from the legacy app to the Stage, add an empty Inspector slot, gate the inspector hook on a subscriber/u URL param. Tabs and actual Subscribers/Settings content land in PR 3 / PR 4.
• PR 3 — Port Subscribers DataViews behind flag. #48581 — Bring Subscribers UI under _inc/subscribers/ (porting from the prototype branch). Lift Gravatar into @automattic/jetpack-components/gravatar. Single route, no tab UI yet.
• PR 4 — Settings tab + tabs UI behind flag. #48607 — Single / route with ?tab= driving Tabs.Root from @wordpress/ui; persistent tabs so the active-tab indicator slides. Mount the existing 9 settings sections inside the Settings tab unchanged (legacy primitives). Page chrome polish: sticky header, opaque background, full-width tab-row separator.
• PR 5 — Settings on WPDS Card primitives. #48608 — Convert all 9 sections to Card.Root / Card.Header / Card.Title / Card.Content from @wordpress/ui. ExternalLink → WPDS Link openInNewTab; legacy Button save buttons opt into __next40pxDefaultSize. Pure refactor inside the flagged chassis.
• PR 6 — Flip the flag default + retire legacy. #48613 — Default the modernization filter to true (or remove the gate entirely). Retire the standalone Subscribers wp-admin menu and the "Subscribers ↗" Calypso shortcut. Remove the subscribers-dashboard package. Add jetpack_newsletter_tab_view Tracks event; lift analytics init to the page Stage; ARIA Tabs verified; RTL pass.

Test plan (per PR)

• Hard-reload wp-admin/admin.php?page=jetpack-newsletter with the filter OFF — legacy Newsletter Settings page is unchanged at every step.
• Add add_filter( 'rsm_jetpack_ui_modernization_newsletter', '__return_true' ); and reload — the new chassis renders, growing in capability PR-by-PR.
• Toggle the filter back off — legacy returns immediately, no cache clears.
• PR 6 removes the gate entirely and the legacy surface; the test plan there is the smoke test for the unified product.

Architecture decisions (carried from #48416)

• Where it lives. The unified product lives in projects/packages/newsletter/. The package keeps its name and menu slug (jetpack-newsletter).
• Scaffolding source of truth. wp-build pipeline (alongside webpack until PR 6); Page from @wordpress/admin-ui; @wordpress/route for tabs; React Query; WP_Build_Polyfills; the boot-layout stage / inspector pattern.
• Tabs. Tabs from @wordpress/ui, route-driven via ?tab=, persistent Tabs.Root so the indicator slides.
• Settings keeps its 9 sections 1:1. The card primitive moves to WPDS Card.* from @wordpress/ui. Structural cleanup is a follow-up.
• Menu changes. Standalone Subscribers menu retires. Newsletter is the single entry.
• Filter compatibility. jetpack_wp_admin_subscriber_management_enabled continues to gate the in-admin Subscribers experience inside the Newsletter page.
• Backend. REST proxies (/wpcom/v2/subscribers/*) and the existing Newsletter settings REST API are preserved as-is.

Out of scope

• Restructuring the 9 settings sections.
• Adding new settings, new Subscribers features, or migrating REST proxies.
• Touching the Newsletter block or any editor-side code.

References

• Prototype umbrella (superseded): #48416
• Prototype PR (superseded, kept as working reference): #48420
• VideoPress wp-build scaffold (PR 1 cadence model): #48494
• Subscribers DataViews port: #48386
• Forms tabs reference: projects/packages/forms/src/dashboard/components/forms-responses-tabs/index.tsx
• WPDS Card / Tabs: @wordpress/ui