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/shared/Monitor.Protocol/Rpc/MonitorRpc.cs

@@ -152,6 +152,10 @@ public sealed class ProbeAgentsParams
     /// <summary>Probe 目前持有的列表版本（用于调试；服务端总是返回当前列表）。</summary>
     [JsonPropertyName("have_version")]
     public long HaveVersion { get; set; }
+
+    /// <summary>Probe 的系统和架构（只用于展示；Agent 记在会话上，随连接状态报给 Backend）。</summary>
+    [JsonPropertyName("platform")]
+    public PlatformInfo? Platform { get; set; }
 }
 
 public sealed class ProbeAgentsResult
@@ -194,9 +198,41 @@ public sealed class LocalLimits
     public int MaxConcurrentTests { get; set; } = 1;
 }
 
+/// <summary>
+/// 节点的系统和架构（粗粒度，只用于展示）：<c>os</c> 是 linux / windows / macos / other，<c>name</c> 是「Debian 12」「Windows 11」这样的名字，
+/// <c>arch</c> 是 x64 / arm64 / arm / x86。<b>没有主机名、内核版本、具体型号</b>。另外带着这个程序自己的构建信息（版本、tag、构建时间）。
+/// </summary>
+public sealed class PlatformInfo
+{
+    [JsonPropertyName("os")]
+    public string Os { get; set; } = "other";
+
+    [JsonPropertyName("name")]
+    public string? Name { get; set; }
+
+    [JsonPropertyName("arch")]
+    public string? Arch { get; set; }
+
+    /// <summary>程序的版本、tag、提交（短）和构建时间（UTC，ISO 8601）：<see cref="BuildInfo"/> 构建时写进去的，没设置就是空。</summary>
+    [JsonPropertyName("version")]
+    public string? Version { get; set; }
+
+    [JsonPropertyName("tag")]
+    public string? Tag { get; set; }
+
+    [JsonPropertyName("commit")]
+    public string? Commit { get; set; }
+
+    [JsonPropertyName("built_at")]
+    public string? BuiltAt { get; set; }
+}
+
 /// <summary><c>fleet.hello</c> 的附加字段。</summary>
 public sealed class AgentHelloMeta
 {
+    [JsonPropertyName("platform")]
+    public PlatformInfo? Platform { get; set; }
+
     [JsonPropertyName("tls_spki")]
     public string? TlsSpki { get; set; }
 
@@ -266,6 +302,9 @@ public sealed class ConnectedProbe
 
     [JsonPropertyName("since_utc_ms")]
     public long SinceUtcMs { get; set; }
+
+    [JsonPropertyName("platform")]
+    public PlatformInfo? Platform { get; set; }
 }
 
 /// <summary><c>fleet.heartbeat</c> 的附加字段：只有进程内的计数，没有主机信息。</summary>
@@ -434,6 +473,7 @@ public static class FragmentKeys
 [JsonSerializable(typeof(ProbeAgentsParams))]
 [JsonSerializable(typeof(ProbeAgentsResult))]
 [JsonSerializable(typeof(AgentEnrollMeta))]
+[JsonSerializable(typeof(PlatformInfo))]
 [JsonSerializable(typeof(AgentHelloMeta))]
 [JsonSerializable(typeof(AgentHeartbeatExtra))]
 [JsonSerializable(typeof(RawChunkHeader))]