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/src/Mbproxy/Admin/wwwroot/detail.js

@@ -7,9 +7,20 @@
 
 (function () {
   // ── PLC name from the URL path: /plc/{name} ────────────────────────────
-  const plcName = decodeURIComponent(location.pathname.replace(/^\/plc\//, ''));
+  // decodeURIComponent throws URIError on a malformed %-escape; fall back to the
+  // raw path segment and flag the failure so the boot path shows a notice instead
+  // of letting the whole script abort on the very first statement.
+  const rawSegment = location.pathname.replace(/^\/plc\//, '');
+  let plcName, plcNameError = false;
+  try {
+    plcName = decodeURIComponent(rawSegment);
+  } catch {
+    plcName = rawSegment;
+    plcNameError = true;
+  }
 
   const $ = (id) => document.getElementById(id);
+  const { escapeHtml } = window.mbproxyUtil;
 
   document.title = `mbproxy — ${plcName}`;
   $('crumb-name').textContent = plcName;
@@ -20,9 +31,6 @@
     if (n === null || n === undefined) return '—';
     return n.toLocaleString('en-US');
   }
-  function escapeHtml(s) {
-    return String(s).replace(/[&<>]/g, c => ({ '&': '&amp;', '<': '&lt;', '>': '&gt;' }[c]));
-  }
   function hex4(n) { return '0x' + (n & 0xffff).toString(16).toUpperCase().padStart(4, '0'); }
 
   // First debug-row cell: the tag's friendly name (when configured) over its PDU
@@ -37,7 +45,7 @@
   }
 
   function formatAge(sec) {
-    if (sec === null || sec === undefined) return '—';
+    if (!Number.isFinite(sec) || sec < 0) return '—';
     if (sec < 1) return 'now';
     if (sec < 60) return sec.toFixed(1) + 's';
     const m = Math.floor(sec / 60);
@@ -299,5 +307,12 @@
     }
   }
 
-  document.addEventListener('DOMContentLoaded', connect);
+  document.addEventListener('DOMContentLoaded', () => {
+    if (plcNameError) {
+      showNotice('The PLC name in this URL could not be decoded — the address is ' +
+                 'malformed. Return to the fleet page and open the PLC from the table.');
+      return;
+    }
+    connect();
+  });
 })();