Added trial private site UI lockdown for Access settings

[ErisDS]

Summary

• When the publicSiteAccess host limit is active, the Access settings page shows a gradient "Pre-launch mode" banner with an upgrade CTA, disables the site visibility select (locked to Private), disables the access code text field, and exposes a regenerate button for the access code
• Two separate signals gate the UI: the publicSiteAccess limit (via useLimiter) drives trial-specific messaging (banner, CTA), while isSettingReadOnly drives generic field disabling — so trial copy won't appear if settings are ever marked read-only for a different reason
• Orange "Private" badge with lock icon appears on the Access nav item in the settings sidebar and next to the site title in the main admin sidebar header whenever the site is private
• Improved disabled styling for Select and TextField in the design system — uses muted backgrounds and subtle rings instead of opacity wash
• Added publicSiteAccess to the host settings limits type definition

Lifts the UI from the spike on codex/trial-private-site-simulator-demo, wired to real limit and settings signals instead of the localStorage simulator.

Depends on #27917 (merged) for the is_read_only field on settings. The regenerate endpoint is a follow-up (sends password: null via Settings edit for now).

Test plan

• With publicSiteAccess limit active: banner shown, visibility select disabled and locked to Private, access code field disabled, regenerate button visible, RSS hint hidden
• Without the limit: all controls editable, no banner, no regenerate button change in behaviour
• Private badge visible in settings sidebar and admin sidebar header when site is private
• Design system: disabled Select and TextField render with muted bg + ring, no opacity wash
• Existing acceptance tests pass (79 passed, 0 failures)

🤖 Generated with Claude Code

[coderabbitai]

Walkthrough

This PR updates the form component disabled styling and implements private site access control across the admin interface. It refines the Select and TextField components to use explicit text colors and shadow effects instead of opacity for disabled states. The membership access UI adds a locked state that prevents modification when is_private or password settings are read-only, disables relevant controls, and provides an async regenerate access code action. The sidebar and app header now display a visual "Private" badge when the site is set to private mode, providing users with a clear indicator of the current access setting.

Possibly related PRs

• TryGhost/Ghost#27917: Marks is_private and password settings as read-only in the Settings API to enforce pre-launch mode constraints, which directly aligns with this PR's locked state implementation that disables these controls when read-only.
• TryGhost/Ghost#27920: Adds boot-time enforcement and handling for publicSiteAccess and access-code generation/limits, which relates to this PR's UI and config typing changes for publicSiteAccess.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: adding trial private site UI lockdown for Access settings, which is reflected throughout the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The pull request description clearly outlines the changes: trial UI lockdown for Access settings, Private badge additions, design system improvements, and detailed test plan with specific scenarios.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch add-trial-private-site-admin-ui

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