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/AgentEndToEndTests.cs

@@ -256,7 +256,7 @@ public sealed class AgentEndToEndTests
         string? startFailure = null;
         for (int attempt = 0; attempt < 5; attempt++)
         {
-            accepted = await _backend.Admin.PostAsync($"/api/admin/lines/{agent.Id}~{probe.ProbeId}/tests", BackendFixture.Json(new JsonObject { ["plan"] = "hy2_ladder", ["params"] = new JsonObject { ["size"] = 1000 } }));
+            accepted = await _backend.Admin.PostAsync($"/api/admin/lines/{agent.Id}~{probe.ProbeId}/tests", BackendFixture.Json(new JsonObject { ["plan"] = "hy2_ladder", ["params"] = new JsonObject { ["size"] = 1000, ["duration_s"] = 5 } }));
             if (accepted.StatusCode == System.Net.HttpStatusCode.Accepted)
             {
                 break;
@@ -291,7 +291,7 @@ public sealed class AgentEndToEndTests
         Assert.Equal(4, probe.Phases.Count);
         Assert.All(probe.Phases, phase => Assert.Equal("udp_train", phase.Kind));
 
-        // 单块最多 50000 条记录：50 和 100 Mbit/s 阶段（10 s、1000 B 的包）各被 Probe 切成多个列块
+        // 单块最多 50000 条记录：50 和 100 Mbit/s 阶段（5 s、1000 B 的包）里 100 Mbit/s 的那一段有 6 万多条记录，被 Probe 切成两个列块
         Assert.True(agent.App.Counters.Fragments > 4);
         await BackendFixture.EventuallyAsync(
             async () => (await _backend.GetJsonAsync(_backend.Admin, $"/api/admin/raw/chunks?agent_id={agent.Id}"))["items"]!.AsArray().Count(c => c!["kind"]!.GetValue<string>() == "udp_train") == agent.App.Counters.Fragments,