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/shared/tests/Monitor.Measurement.Tests/PlatformDetectorTests.cs

@@ -0,0 +1,27 @@
+namespace Monitor.Measurement.Tests;
+
+public sealed class PlatformDetectorTests
+{
+    [Theory]
+    [InlineData("PRETTY_NAME=\"Debian GNU/Linux 12 (bookworm)\"|NAME=\"Debian GNU/Linux\"|VERSION_ID=\"12\"|ID=debian", "Debian 12")]
+    [InlineData("NAME=\"OpenWrt\"|VERSION_ID=\"24.10.0\"|ID=openwrt", "OpenWrt 24.10")]
+    [InlineData("NAME=\"Alpine Linux\"|ID=alpine|VERSION_ID=3.22.1", "Alpine Linux 3.22")]
+    [InlineData("NAME=\"Ubuntu\"|VERSION_ID=\"24.04\"", "Ubuntu 24.04")]
+    [InlineData("ID=arch", "arch")]
+    [InlineData("", "Linux")]
+    public void LinuxName_IsTheDistroAndItsMajorMinorVersion(string osRelease, string expected)
+    {
+        Assert.Equal(expected, PlatformDetector.LinuxName(osRelease.Split('|', StringSplitOptions.RemoveEmptyEntries)));
+    }
+
+    [Fact]
+    public void Detect_ReportsOnlyCoarseInformation()
+    {
+        var platform = PlatformDetector.Detect();
+        Assert.Contains(platform.Os, new[] { "linux", "windows", "macos", "other" });
+        Assert.Contains(platform.Arch, new[] { "x64", "arm64", "arm", "x86" });
+        // 没有主机名、内核版本：名字很短，也不含小数点以外的版本细节
+        Assert.True((platform.Name ?? string.Empty).Length < 40);
+        Assert.DoesNotContain(Environment.MachineName, platform.Name ?? string.Empty, StringComparison.OrdinalIgnoreCase);
+    }
+}