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/sim/mbproxy.smoke.config.json

@@ -1,5 +1,6 @@
-// mbproxy smoke-test configuration — used by the Phase 4/5 web-UI browser smoke
-// tests (see plans/2026-05-15-webui-dashboard.md). NOT a deployment config.
+// mbproxy smoke-test configuration for the web-UI browser smoke tests.
+// NOT a deployment config. The Resilience and Cache sections are intentionally
+// omitted — the smoke run relies on their built-in defaults.
 //
 // Topology:
 //   * line-a / line-b  → the dl205 simulator on 127.0.0.1:5020 (run-dl205-sim.ps1).