* Fall back to display name when a published-post email has no username.
Facebook Pages often have no vanity username, which left the email as "Facebook Page (@)". Use username, then display name, and omit the empty parentheses when both are missing.
* Move the notification account label onto PostPlatform.
The fallback belongs on the row that always has a platform, so mail and in-app notify can call one method instead of guarding a missing social account at every call site.
* Cover the in-app published/failed notification body for Facebook Pages.
The email tests were not enough — SendNotification body is what the in-app inbox shows, and that path also goes through notificationLabel().
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