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/ApplicationServiceCollectionExtensions.cs

@@ -27,6 +27,7 @@ public static class ApplicationServiceCollectionExtensions
         services.AddSingleton<AgentService>();
         services.AddSingleton<AgentEventService>();
         services.AddSingleton<HostStatusService>();
+        services.AddSingleton<NodeInfoService>();
         services.AddSingleton<ProbeService>();
         services.AddSingleton<LineService>();
         services.AddSingleton<TestService>();