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/app/dashboard/traces/[id]/page.tsx

@@ -0,0 +1,87 @@
+import { notFound } from "next/navigation";
+import { TraceDetail } from "@/components/trace-detail";
+
+interface TraceResponse {
+  trace: {
+    id: string;
+    name: string;
+    status: "RUNNING" | "COMPLETED" | "ERROR";
+    startedAt: string;
+    endedAt: string | null;
+    durationMs: number | null;
+    tags: string[];
+    metadata: Record<string, unknown>;
+    costUsd: number | null;
+  };
+  decisionPoints: Array<{
+    id: string;
+    type: string;
+    chosenAction: string;
+    alternatives: string[];
+    reasoning: string | null;
+    contextSnapshot: Record<string, unknown> | null;
+    confidence: number | null;
+    timestamp: string;
+  }>;
+  spans: Array<{
+    id: string;
+    name: string;
+    type: string;
+    status: "OK" | "ERROR" | "CANCELLED";
+    startedAt: string;
+    endedAt: string | null;
+    durationMs: number | null;
+    input: unknown;
+    output: unknown;
+    metadata: Record<string, unknown>;
+  }>;
+  events: Array<{
+    id: string;
+    type: string;
+    name: string;
+    timestamp: string;
+    metadata: Record<string, unknown>;
+  }>;
+}
+
+async function getTrace(id: string): Promise<TraceResponse | null> {
+  try {
+    const res = await fetch(`http://localhost:3000/api/traces/${id}`, {
+      cache: "no-store",
+    });
+
+    if (!res.ok) {
+      if (res.status === 404) {
+        return null;
+      }
+      throw new Error(`Failed to fetch trace: ${res.status}`);
+    }
+
+    return res.json();
+  } catch (error) {
+    console.error("Error fetching trace:", error);
+    return null;
+  }
+}
+
+interface TraceDetailPageProps {
+  params: Promise<{ id: string }>;
+}
+
+export default async function TraceDetailPage({ params }: TraceDetailPageProps) {
+  const { id } = await params;
+  const data = await getTrace(id);
+
+  if (!data) {
+    notFound();
+  }
+
+  return (
+    <TraceDetail
+      trace={data.trace}
+      decisionPoints={data.decisionPoints}
+      spans={data.spans}
+      events={data.events}
+    />
+  );
+}