Enforce the 13+ age requirement with Apple's Declared Age Range API

[crazytonyli]

Description

Fixes https://linear.app/a8c/issue/CMM-2393

Apple's Declared Age Range API tells apps which age range the App Store account belongs to, in the regions where age assurance laws apply. Our terms require users to be 13 or older, so the app now asks the API once after launch and refuses under-13 accounts.

The main changes:

1. AgeRequirementCoordinator runs one check per process after WindowManager installs the signed-in or signed-out root. Only an affirmative under-13 range restricts. Declined sharing, ineligible regions, unsupported OS versions, and API errors all fail open, so this cannot lock out people the law does not cover.
2. When restricted, AgeRequirementEnforcer shows the sign-in UI, removes the sites that have no WordPress.com account, signs out, and presents an alert. The sign-in UI goes first on purpose: the logout notification handler only shows it when it is not already showing, so this order gives one root transition instead of two.
3. Both sign-in flows check the restriction at entry and again right before persisting the account or site, because the age result can arrive while a sign-in is in progress. A refused sign-in shows the same alert and posts no completion notification.

Every check outcome is tracked as age_requirement_check, without the age range itself.

Testing instructions

The Simulator cannot exercise the Declared Age Range service, so the check is a no-op there. To test the restriction, use a physical device on iOS 26.2 or later with a sandbox Apple Account whose age assurance is set to under 13 (Settings > Developer > Sandbox Apple Account > Manage > Age Assurance).

Regression tests on both sign-in flows are worth doing, since the persistence points in WordPressDotComAuthenticator and SelfHostedSiteAuthenticator changed.

[dangermattic]

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	App Name	Jetpack
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	Build Number	34406
	Version	PR #25979
	Bundle ID	com.jetpack.alpha
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