feat(backend): report node platform, geo and build info

Agents and probes report OS, arch and build (version, tag, commit, time).
The backend looks up country and ISP of public addresses only, stores them
in node_meta (schema v5) and exposes display info via /api/public/nodes,
/api/public/about and /api/admin/build-info behind site.show_build.

backend/probe/src/Monitor.Probe/Link/AgentConnection.cs

@@ -163,10 +163,13 @@ public sealed class AgentConnection
         }
     }
 
+    /// <summary>本机的系统和架构：随列表同步一起告诉 Agent，只用于在面板上展示。</summary>
+    private static readonly PlatformInfo s_platform = PlatformDetector.Detect();
+
     private async Task RefreshDirectoryAsync(RpcSession<ConnectionSession> rpc, CancellationToken ct)
     {
         Result<ProbeAgentsResult> list = await rpc.RequestAsync(
-            MonitorMethods.ProbeAgents, new ProbeAgentsParams { HaveVersion = _env.Directory.Version }, MonitorJsonContext.Default.ProbeAgentsParams, MonitorJsonContext.Default.ProbeAgentsResult, ct);
+            MonitorMethods.ProbeAgents, new ProbeAgentsParams { HaveVersion = _env.Directory.Version, Platform = s_platform }, MonitorJsonContext.Default.ProbeAgentsParams, MonitorJsonContext.Default.ProbeAgentsResult, ct);
         if (list.TryGet(out ProbeAgentsResult? agents, out FleetError? error))
         {
             _env.Directory.Apply(agents);

backend/probe/src/Monitor.Probe/ProbeApp.cs

@@ -11,7 +11,7 @@ namespace Monitor.Probe;
 /// </summary>
 public sealed class ProbeApp : IDisposable
 {
-    public const string Version = "0.1.1";
+    public const string Version = Monitor.Protocol.BuildInfo.Version;
 
     private readonly ConnectionEnv _env;
     private readonly Dictionary<string, Running> _connections = [];