Introduce privacy/telemetry support: add PrivacyConfig and PrivacyAgreementState models, and a PrivacyAgreementService that saves/validates agreement state with HMAC integrity protection (privacy-agreement.state.json). Update AppJsonContext to include new types. Extend OOBE UI (OobeWindow.axaml/.cs) with a Data Location redesign and a new Privacy step (telemetry toggles, telemetry ID, agreement checkbox) and wire up handlers to save privacy-config.json and agreement state. Add a PrivacyPolicyWindow using Markdown.Avalonia to display the privacy policy; add CommunityToolkit.Mvvm and Markdown.Avalonia package references.
