Add Garmin LiveTrack session tracking

A Gmail-watching Apps Script (documented in docs/deployment.adoc) posts
the session ID from each LiveTrack email to a new webhook, which stores
it in data/livetrack.json; a new sidebar panel lists what's been
received.

public/css/style.css

@@ -430,3 +430,33 @@ html, body {
   background: #2a9d8f;
   box-shadow: 0 0 0 2px rgba(42, 157, 143, 0.4);
 }
+
+.livetrack-panel__list {
+  flex: 1;
+  overflow-y: auto;
+  list-style: none;
+  margin: 0;
+  padding: 8px 12px;
+}
+
+.livetrack-panel__item {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+  padding: 6px 0;
+  border-bottom: 1px solid #f0f0f0;
+}
+
+.livetrack-panel__id {
+  font-family: monospace;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.livetrack-panel__time {
+  flex-shrink: 0;
+  color: #888;
+  font-size: 12px;
+}