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/style.css

@@ -1453,13 +1453,74 @@ a:hover { text-decoration: underline; text-shadow: var(--glow-amber); }
 
 /* ── Stale monitoring banner ──────────────────────────────────────── */
 .stale-banner {
-  background: rgba(255, 160, 0, 0.12);
-  border: 1px solid var(--warning);
-  border-left: 4px solid var(--warning);
-  color: var(--warning);
+  background: var(--amber-dim);
+  border: 1px solid var(--amber);
+  border-left: 4px solid var(--amber);
+  color: var(--amber);
   padding: 10px 16px;
-  margin: 12px 16px 0;
+  margin: 12px 0 0;
   font-size: 0.88em;
-  font-family: var(--font-mono);
+  font-family: var(--font);
   border-radius: 2px;
 }
+
+/* ── Link alert badges (error/flap indicators) ────────────────────── */
+.link-alert-badge {
+  display: inline-block;
+  font-size: .6em;
+  font-weight: bold;
+  padding: 1px 5px;
+  border-radius: 2px;
+  background: var(--red-dim);
+  color: var(--red);
+  border: 1px solid var(--red);
+  margin-left: 4px;
+  vertical-align: middle;
+  letter-spacing: .05em;
+}
+
+.link-alert-badge.link-alert-amber {
+  background: var(--amber-dim);
+  color: var(--amber);
+  border-color: var(--amber);
+}
+
+/* ── PoE utilisation bar ──────────────────────────────────────────── */
+.poe-bar-track {
+  height: 3px;
+  background: var(--bg3);
+  border-radius: 2px;
+  margin-top: 3px;
+  overflow: hidden;
+}
+
+.poe-bar-fill {
+  height: 100%;
+  border-radius: 2px;
+  transition: width 0.4s ease;
+}
+
+.poe-bar-ok   { background: var(--green); }
+.poe-bar-warn { background: var(--amber); }
+.poe-bar-crit { background: var(--red); }
+
+/* ── Path mismatch alert ──────────────────────────────────────────── */
+.path-mismatch-alert {
+  background: var(--amber-dim);
+  border-left: 3px solid var(--amber);
+  color: var(--amber);
+  padding: 4px 8px;
+  margin-bottom: 6px;
+  font-size: .72em;
+  border-radius: 2px;
+}
+
+/* ── Error state for data containers ─────────────────────────────── */
+.error-state {
+  padding: 16px 20px;
+  border-left: 3px solid var(--red);
+  background: var(--red-dim);
+  color: var(--red);
+  border-radius: 2px;
+  font-size: .88em;
+}