Refactor error tracking to be edge-triggered with node setters

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/index.html

@@ -1723,7 +1723,7 @@
 
                     const typeEl = document.createElement('div');
                     typeEl.className = 'error-type';
-                    typeEl.textContent = err.type === 'startup' ? 'Present at startup' : 'New errors detected';
+                    typeEl.textContent = 'New errors detected';
                     item.appendChild(typeEl);
                 }