Subscribers: fix layout gap and add SidebarNavigation on JP Cloud

[CGastrell]

Part of JETPACK-1513

Proposed Changes

• Remove extra padding on .layout__content that created a visible gap around the Subscribers DataViews table. Simplify padding to match the masterbar/sidebar boundaries directly.
• Switch <Main> from wideLayout to fullWidthLayout so the page fills the content area.
• Add <SidebarNavigation /> on JP Cloud so mobile users have the hamburger menu to navigate away (was missing — same navigation trap as Earn/Social).
• Replace hardcoded 62px padding-top in the sidebar focus-content rule with var(--masterbar-height, 62px) for consistency.

Why are these changes being made?

The Subscribers page on Calypso had a visible gap between the content area and the sidebar/masterbar boundaries, caused by extra padding in .layout__content. This was inconsistent with other Jetpack admin pages that sit flush against the layout edges.

On JP Cloud mobile, the Subscribers page was missing <SidebarNavigation /> — the same navigation trap we fixed on Earn and Social in #109899. Without the hamburger menu, users had no way to navigate away from the page on mobile.

Testing Instructions

Calypso (WP.com):

• Navigate to /subscribers/<site-slug>
• Verify content sits flush against sidebar and masterbar (no extra gap/padding)
• Verify the DataViews table fills the available viewport height
• On mobile viewport: verify the page works correctly without sidebar

JP Cloud:

• Navigate to /subscribers/<site-slug> on cloud.jetpack.com
• Desktop: verify same flush layout
• Mobile: verify hamburger menu is present (was missing before)

Before / after

Calypso Desktop — gap removed

Before	After

Content now sits flush against sidebar and masterbar. No extra padding gap.

Calypso Mobile

Before	After

JP Cloud Desktop

Before	After

JP Cloud Mobile — navigation trap fixed

Before	After

Before: no hamburger menu, no way to navigate out. After: hamburger + site title present.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations?
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-175480

Jetpack Cloud Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-175480&env=jetpack

Automattic for Agencies Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-175480&env=a8c-for-agencies

Dashboard Live (dotcom) (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-175480&env=dashboard

Dashboard Live (CIAB) (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-175480&env=dashboard-ciab

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• help-center

To test WordPress.com changes, run install-plugin.sh $pluginSlug try/subscribers-layout-normalization on your sandbox.