feat: relative timestamps, message preview, and recency sort in sidebar

- Show relative time (2m, 3h, 1d) next to each session name
- Display last message preview below session name (truncated to 80 chars)
- Sort sessions by most recently updated (within pinned/unpinned groups)
- Map updatedAt and lastMessagePreview from gateway sessions.list response

src/lib/relativeTime.ts

@@ -0,0 +1,19 @@
+/**
+ * Format a timestamp as a short relative time string (e.g. "2m", "3h", "1d").
+ * Returns null for invalid/missing timestamps.
+ */
+export function relativeTime(ts: number | undefined): string | null {
+  if (!ts) return null;
+  const now = Date.now();
+  const diff = Math.max(0, now - ts);
+  const seconds = Math.floor(diff / 1000);
+  if (seconds < 60) return '<1m';
+  const minutes = Math.floor(seconds / 60);
+  if (minutes < 60) return `${minutes}m`;
+  const hours = Math.floor(minutes / 60);
+  if (hours < 24) return `${hours}h`;
+  const days = Math.floor(hours / 24);
+  if (days < 30) return `${days}d`;
+  const months = Math.floor(days / 30);
+  return `${months}mo`;
+}