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/ProbeEndToEndTests.cs

@@ -64,7 +64,7 @@ public sealed class ProbeEndToEndTests
         await using ProbeHarness probe = await ProbeHarness.StartAsync(_backend);
         await BackendFixture.EventuallyAsync(async () => (await AgentViewAsync(agent))["probe_connected"]!.GetValue<bool>());
 
-        using HttpResponseMessage accepted = await _backend.Admin.PostAsync($"/api/admin/lines/{agent.Id}~{probe.ProbeId}/tests", BackendFixture.Json(new JsonObject { ["plan"] = "hy2_ladder", ["params"] = new JsonObject { ["size"] = 1000 } }));
+        using HttpResponseMessage accepted = await _backend.Admin.PostAsync($"/api/admin/lines/{agent.Id}~{probe.ProbeId}/tests", BackendFixture.Json(new JsonObject { ["plan"] = "hy2_ladder", ["params"] = new JsonObject { ["size"] = 1000, ["duration_s"] = 5 } }));
         Assert.True(accepted.StatusCode == System.Net.HttpStatusCode.Accepted, await accepted.Content.ReadAsStringAsync());
         string testId = JsonNode.Parse(await accepted.Content.ReadAsStringAsync())!["id"]!.GetValue<string>();
 
@@ -78,7 +78,7 @@ public sealed class ProbeEndToEndTests
             TimeSpan.FromSeconds(90));
         Assert.True(test["status"]!.GetValue<string>() == "succeeded", test.ToJsonString());
 
-        await BackendFixture.EventuallyAsync(async () => await ChunkCountAsync(agent, "udp_train") >= 5, because: "50 and 100 Mbit/s phases are split into several fragments");
+        await BackendFixture.EventuallyAsync(async () => await ChunkCountAsync(agent, "udp_train") >= 5, because: "the 100 Mbit/s phase (5 s of 1000 B packets, over 50000 records) is split into two fragments");
         Assert.Equal(0, agent.App.Counters.FragmentsDiscarded);
     }