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/tests/Monitor.Backend.Tests/BackendFixture.cs

@@ -192,6 +192,8 @@ public class BackendFixture : IAsyncLifetime
                 options.InitialAdminUser = AdminUser;
                 options.InitialAdminPassword = AdminPassword;
                 options.LoginAttemptsPerMinute = LoginAttemptsPerMinute;
+                options.GeoLookupUrl = string.Empty; // 测试里不访问外部服务
+                options.PhaseDurationScale = 0.2; // 端到端测试里 connect、hold 这些固定 30 秒的阶段缩成 6 秒
             });
         await App.StartAsync();
         string address = App.Services.GetRequiredService<IServer>().Features.Get<IServerAddressesFeature>()!.Addresses.First();