Add privacy agreement UI, models, and service

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.

LanMountainDesktop.Launcher/Models/PrivacyAgreementState.cs

@@ -0,0 +1,42 @@
+namespace LanMountainDesktop.Launcher.Models;
+
+/// <summary>
+/// 隐私协议同意状态模型（带防篡改保护）
+/// </summary>
+public class PrivacyAgreementState
+{
+    /// <summary>
+    /// 用户是否同意隐私协议
+    /// </summary>
+    public bool IsAgreed { get; set; } = false;
+
+    /// <summary>
+    /// 同意时间（UTC）
+    /// </summary>
+    public DateTime AgreedAtUtc { get; set; }
+
+    /// <summary>
+    /// 同意的协议版本
+    /// </summary>
+    public string AgreementVersion { get; set; } = "1.0";
+
+    /// <summary>
+    /// 用户标识（匿名）
+    /// </summary>
+    public string UserId { get; set; } = string.Empty;
+
+    /// <summary>
+    /// 设备标识
+    /// </summary>
+    public string DeviceId { get; set; } = string.Empty;
+
+    /// <summary>
+    /// 数据完整性校验哈希（HMAC-SHA256）
+    /// </summary>
+    public string IntegrityHash { get; set; } = string.Empty;
+
+    /// <summary>
+    /// 用于生成哈希的随机盐值
+    /// </summary>
+    public string Salt { get; set; } = string.Empty;
+}