[2.x] fix: send transactional emails in recipient's preferred locale

[imorland]

Summary

• Translates email subject/body under the recipient's preferred locale (User::getPreference('locale')), falling back to the forum's default_locale.
• Carries the per-recipient locale through to SendInformationalEmailJob and SendAbandonedExtensionsEmailJob so the wrapper template's greeting / signoff strings render in the same language as subject/body — fixing the mixed-language output described in [2.x] Transactional emails are sent in mixed languages instead of the recipient's preferred locale #4626.
• Moves abandoned-extensions translation inside the per-admin loop so each admin receives the email in their own locale (was previously translated once outside the loop, so all admins got the same language).
• Mirrors the existing pattern in NotificationMailer::send().

Fixes #4626

Changes

• Mail/Job/SendInformationalEmailJob — adds optional ?string $locale (after $views to preserve positional BC); injects TranslatorInterface into handle() and applies locale before send.
• Mail/Job/SendAbandonedExtensionsEmailJob — same shape.
• Api/Controller/SendTestMailController — switches to actor's locale, translates, restores previous locale in finally.
• User/Job/RequestPasswordResetJob — same pattern, recipient = looked-up user.
• User/EmailConfirmationMailer — same pattern.
• User/AccountActivationMailerTrait — same pattern (already had User $user in scope).
• Extension/AbandonedExtensionsFetcher::notifyAdmins() — moves translation inside loop, switches to Flarum's Locale\TranslatorInterface (Symfony's contract doesn't expose setLocale()), wraps loop in try/finally.

Test plan

• Forum default locale = German (de), admin user preference = English (en). Trigger Send test mail — full email renders in English.
• Same setup, request a password reset for the admin — full email renders in English.
• User with no locale preference set — email renders in the forum default locale (German).
• Two admins with different locale preferences (e.g. one English, one German). Trigger an abandoned-extensions notification — each admin receives the email in their own locale.
• Verify subscription / reply notification mails (handled by NotificationMailer) are unchanged.
• PHPUnit suite passes.

Notes

• Backward compatibility: the new locale parameter on both jobs is optional and trailing, so existing third-party callers (positional or named) continue to work unchanged. When null, behavior is identical to today.
• All changes target a single bug class — no unrelated cleanup.