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/PrivacyConfig.cs

@@ -0,0 +1,22 @@
+namespace LanMountainDesktop.Launcher.Models;
+
+/// <summary>
+/// 隐私配置模型
+/// </summary>
+public class PrivacyConfig
+{
+    /// <summary>
+    /// 是否启用崩溃报告遥测
+    /// </summary>
+    public bool CrashTelemetryEnabled { get; set; } = true;
+
+    /// <summary>
+    /// 是否启用使用统计遥测
+    /// </summary>
+    public bool UsageTelemetryEnabled { get; set; } = true;
+
+    /// <summary>
+    /// 隐私追踪 ID
+    /// </summary>
+    public string TelemetryId { get; set; } = string.Empty;
+}