mbproxy: close out the dashboard code-review minor findings

Resolves the remaining Minor items from the 2026-05-15 review so the
web-UI dashboard work has no open follow-ups: a real-HubConnection
end-to-end test for the SignalR feed, stable mbproxy.admin.broadcast.*
log-event names, keyboard/aria accessibility on the fleet table,
frontend JS hardening (URL-decode guard, NaN guards, shared util.js),
reconciler<->capture-registry coverage, throwing-sink and embedded-asset
tests, broadcaster polish, and a soft upper bound on AdminPushIntervalMs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/tests/Mbproxy.Tests/Admin/AdminEndpointTests.cs

@@ -243,7 +243,22 @@ public sealed class AdminEndpointTests
         response.Headers.CacheControl?.ToString().ShouldContain("immutable");
 
         var bytes = await response.Content.ReadAsByteArrayAsync(TestContext.Current.CancellationToken);
-        bytes.Length.ShouldBeGreaterThan(0);
+
+        // The served bytes must be the actual embedded asset — not some other resource
+        // of the same length. Compare against the manifest resource directly.
+        byte[] expected = ReadEmbeddedAsset(file);
+        bytes.ShouldBe(expected, $"GET /assets/{file} must return the embedded asset verbatim");
+    }
+
+    /// <summary>Reads a <c>wwwroot</c> asset straight from the assembly's manifest resources.</summary>
+    private static byte[] ReadEmbeddedAsset(string fileName)
+    {
+        using var stream = typeof(Mbproxy.Admin.StatusHub).Assembly
+            .GetManifestResourceStream("Mbproxy.Admin.wwwroot." + fileName)
+            ?? throw new InvalidOperationException($"Embedded asset not found: {fileName}");
+        using var ms = new MemoryStream();
+        stream.CopyTo(ms);
+        return ms.ToArray();
     }
 
     [Fact(Timeout = 5_000)]