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/Configuration/ReloadValidatorTests.cs

@@ -366,4 +366,35 @@ public sealed class ReloadValidatorTests
         Assert.False(valid);
         Assert.Contains(errors, e => e.Contains("AdminPushIntervalMs"));
     }
+
+    [Fact]
+    public void Validate_AdminPushIntervalMs_AboveUpperBound_Fails()
+    {
+        // The soft upper bound (60 s) catches a seconds-as-milliseconds typo that
+        // would make the "live" dashboard feed effectively non-live.
+        var opts = new MbproxyOptions
+        {
+            Plcs                = [MakePlc("PLC-A", 5020)],
+            AdminPushIntervalMs = 60_001,
+        };
+
+        bool valid = ReloadValidator.Validate(opts, out var errors);
+
+        Assert.False(valid);
+        Assert.Contains(errors, e => e.Contains("AdminPushIntervalMs"));
+    }
+
+    [Fact]
+    public void Validate_AdminPushIntervalMs_AtUpperBound_Passes()
+    {
+        var opts = new MbproxyOptions
+        {
+            Plcs                = [MakePlc("PLC-A", 5020)],
+            AdminPushIntervalMs = 60_000,
+        };
+
+        bool valid = ReloadValidator.Validate(opts, out var errors);
+
+        Assert.True(valid, string.Join("; ", errors));
+    }
 }