Cancel in-flight diagnostic poll when user selects a new port

Previously switching ports while a diagnostic was running left the
setInterval timer active, causing the result to be written into the
old (now detached) DOM elements and never shown to the user.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/inspector.html

@@ -218,6 +218,7 @@ let _apiData        = null;
 function selectPort(el) {
   const swName = el.dataset.switch;
   const idx    = parseInt(el.dataset.portIdx, 10);
+  if (_diagPollTimer) { clearInterval(_diagPollTimer); _diagPollTimer = null; }
   document.querySelectorAll('.switch-port-block.selected')
     .forEach(e => e.classList.remove('selected'));
   el.classList.add('selected');