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/src/Monitor.Backend.Application/Agents/AgentRuntime.cs

@@ -5,7 +5,7 @@ using Monitor.Protocol;
 namespace Monitor.Backend.Application;
 
 /// <summary>Agent 眼中的一个已连接 Probe（Agent 在事件和心跳里上报）。</summary>
-public sealed record ConnectedProbeInfo(string Id, string? Version, string? Remote, long SinceMs);
+public sealed record ConnectedProbeInfo(string Id, string? Version, string? Remote, long SinceMs, PlatformInfo? Platform = null);
 
 /// <summary>Agent 的运行时状态（只在内存中）：连着哪些 Probe、最近的进程内计数、端点变化次数。</summary>
 public sealed class AgentRuntimeState