feat: node info, build info and acrylic web theme

Squash of recent dev work: return_session login, backend switcher, line
detail timeline, overview strips, node platform/country/ISP info with flags
and icons, build info (version, tag, time) behind site.show_build, and the
acrylic display theme.

backend/agent/src/Monitor.Agent/AgentApp.cs

@@ -12,7 +12,7 @@ namespace Monitor.Agent;
 /// <summary>Agent 的组装点：数据面、点对点监听、与 Backend 的链路都在这里接起来。没有全局状态，测试可以在同一进程里启动多个。</summary>
 public sealed class AgentApp : IDisposable
 {
-    public const string Version = "0.1.1";
+    public const string Version = Monitor.Protocol.BuildInfo.Version;
 
     private readonly AgentConfig _config;
     private readonly LoadedNodeIdentity _identity;
@@ -244,6 +244,7 @@ public sealed class AgentApp : IDisposable
     {
         var meta = new AgentHelloMeta
         {
+            Platform = Monitor.Measurement.PlatformDetector.Detect(),
             TlsSpki = _identity.File.TlsSpki,
             ProbeTcpPort = ControlPort,
             ProbeUdpPorts = [.. UdpPorts],

backend/agent/src/Monitor.Agent/DataPlane/Registries.cs

@@ -64,6 +64,9 @@ public sealed class ProbeSession : DataSession
 
     public long ConnectedAtUtcMs { get; init; }
 
+    /// <summary>Probe 自报的系统和架构（随列表同步一起来，只用于展示）；还没报过就是 null。</summary>
+    public PlatformInfo? Platform { get; set; }
+
     public RpcSession<PeerContext>? Rpc { get; set; }
 
     /// <summary>最近一次认证通过的 UDP 包的源地址（Agent 把单向数据发往这里）。</summary>

backend/agent/src/Monitor.Agent/Peer/PeerHub.cs

@@ -220,7 +220,7 @@ public sealed class PeerHub
     /// <summary>当前连着的 Probe（心跳和 Backend 重连之后的补报都用它）。</summary>
     public IReadOnlyList<ConnectedProbe> ConnectedProbes() => [.. _sessions.All.Select(Describe)];
 
-    private static ConnectedProbe Describe(ProbeSession p) => new() { Id = new ProbeId(p.ProbeId), Version = p.Version, Remote = p.RemoteAddress, SinceUtcMs = p.ConnectedAtUtcMs };
+    private static ConnectedProbe Describe(ProbeSession p) => new() { Id = new ProbeId(p.ProbeId), Version = p.Version, Remote = p.RemoteAddress, SinceUtcMs = p.ConnectedAtUtcMs, Platform = p.Platform };
 
     public IReadOnlyList<string> ConnectedProbeIds() => [.. _sessions.All.Select(p => p.ProbeId)];
 
@@ -306,7 +306,15 @@ public sealed class PeerHub
                 MonitorMethods.ProbeAgents,
                 MonitorJsonContext.Default.ProbeAgentsParams,
                 MonitorJsonContext.Default.ProbeAgentsResult,
-                (_, request, ct) => uplink.RequestProbeAgentsAsync(request, ct))
+                (context, request, ct) =>
+                {
+                    if (request.Platform is { } platform)
+                    {
+                        context.Session.Platform = platform;
+                    }
+
+                    return uplink.RequestProbeAgentsAsync(request, ct);
+                })
             .On(
                 MonitorMethods.Clock,
                 MonitorJsonContext.Default.ClockParams,