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/EnrollmentAndTaskSetTests.cs

@@ -83,6 +83,14 @@ public sealed class EnrollmentAndTaskSetTests
         using HttpResponseMessage revoke = await _backend.Admin.DeleteAsync("/api/admin/join-tokens/" + id);
         Assert.Equal(HttpStatusCode.NoContent, revoke.StatusCode);
         await Assert.ThrowsAsync<InvalidOperationException>(() => AgentSimulator.EnrollAsync(_backend.Anonymous, revoked, _backend.UniqueName("revoked")));
+
+        // 吊销之后记录还在（显示「已吊销」）；purge=true 才是彻底删除
+        tokens = await _backend.GetJsonAsync(_backend.Admin, "/api/admin/join-tokens");
+        Assert.Contains(tokens["items"]!.AsArray(), t => t!["id"]!.GetValue<string>() == id);
+        using HttpResponseMessage purge = await _backend.Admin.DeleteAsync($"/api/admin/join-tokens/{id}?purge=true");
+        Assert.Equal(HttpStatusCode.NoContent, purge.StatusCode);
+        tokens = await _backend.GetJsonAsync(_backend.Admin, "/api/admin/join-tokens");
+        Assert.DoesNotContain(tokens["items"]!.AsArray(), t => t!["id"]!.GetValue<string>() == id);
     }
 
     [Fact]