feat: UI improvements — event ages, error badges, PoE bars, mismatch detection

- events table: add Last Seen column; show relative times ("3h ago") with
  absolute timestamp on hover; update updateEventsTable() in app.js to match
- links.html: add error/drop/flap alert badges to interface and port card headers
- links.html: PoE power bar (draw/max ratio with colour-coded fill) and poe_mode
- links.html: stale data warning banner when link_stats are >2 minutes old
- links.html: improved error handler shows HTTP status instead of generic message
- links.html: fix collapse state persisted to localStorage (was sessionStorage,
  lost on browser restart); fix collapseAll/expandAll to also persist state
- inspector.html: duplex mismatch and speed mismatch warnings in path debug panel
- inspector.html: carrier changes added to server column of path debug
- style.css: new classes — .link-alert-badge, .poe-bar-*, .path-mismatch-alert,
  .error-state; fix .stale-banner to use CSS variables

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

static/app.js

@@ -172,7 +172,8 @@ function updateEventsTable(events) {
         <td><strong>${escHtml(e.target_name)}</strong></td>
         <td>${escHtml(e.target_detail || '–')}</td>
         <td class="desc-cell" title="${escHtml(e.description || '')}">${escHtml((e.description||'').substring(0,60))}${(e.description||'').length>60?'…':''}</td>
-        <td class="ts-cell">${escHtml(e.first_seen||'')}</td>
+        <td class="ts-cell" title="${escHtml(e.first_seen||'')}">${fmtRelTime(e.first_seen)}</td>
+        <td class="ts-cell" title="${escHtml(e.last_seen||'')}">${fmtRelTime(e.last_seen)}</td>
         <td>${e.consecutive_failures}</td>
         <td>${ticket}</td>
         <td>
@@ -190,7 +191,7 @@ function updateEventsTable(events) {
         <thead>
           <tr>
             <th>Sev</th><th>Type</th><th>Target</th><th>Detail</th>
-            <th>Description</th><th>First Seen</th><th>Failures</th><th>Ticket</th><th>Actions</th>
+            <th>Description</th><th>First Seen</th><th>Last Seen</th><th>Failures</th><th>Ticket</th><th>Actions</th>
           </tr>
         </thead>
         <tbody>${rows}</tbody>
@@ -300,6 +301,18 @@ document.addEventListener('click', e => {
   }
 });
 
+// ── Relative time ─────────────────────────────────────────────────────
+function fmtRelTime(tsStr) {
+  if (!tsStr) return '–';
+  const d = new Date(tsStr.replace(' UTC', 'Z').replace(' ', 'T'));
+  if (isNaN(d)) return tsStr;
+  const secs = Math.floor((Date.now() - d) / 1000);
+  if (secs < 60)    return `${secs}s ago`;
+  if (secs < 3600)  return `${Math.floor(secs/60)}m ago`;
+  if (secs < 86400) return `${Math.floor(secs/3600)}h ago`;
+  return `${Math.floor(secs/86400)}d ago`;
+}
+
 // ── Utility ───────────────────────────────────────────────────────────
 function escHtml(str) {
   if (str === null || str === undefined) return '';