feat: LangChain auto-instrumentation + dashboard UI

- LangChain: AgentLensCallbackHandler with auto-span creation for
  LLM calls, tool calls, chains, and agent decision logging
- Dashboard: trace list with search, status filters, pagination
- Dashboard: trace detail with Decision/Span/Event tabs
- Dashboard: sidebar layout, responsive design, dark theme

apps/web/src/lib/utils.ts

@@ -0,0 +1,24 @@
+export function formatDuration(ms: number | null | undefined): string {
+  if (ms === null || ms === undefined) return "—";
+  if (ms < 1000) return `${ms}ms`;
+  if (ms < 60000) return `${(ms / 1000).toFixed(1)}s`;
+  return `${(ms / 60000).toFixed(1)}m`;
+}
+
+export function formatRelativeTime(date: string | Date): string {
+  const now = new Date();
+  const then = typeof date === "string" ? new Date(date) : date;
+  const diffMs = now.getTime() - then.getTime();
+  const diffSec = Math.floor(diffMs / 1000);
+  if (diffSec < 60) return "just now";
+  const diffMin = Math.floor(diffSec / 60);
+  if (diffMin < 60) return `${diffMin}m ago`;
+  const diffHour = Math.floor(diffMin / 60);
+  if (diffHour < 24) return `${diffHour}h ago`;
+  const diffDay = Math.floor(diffHour / 24);
+  return `${diffDay}d ago`;
+}
+
+export function cn(...classes: (string | boolean | undefined | null)[]): string {
+  return classes.filter(Boolean).join(" ");
+}