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/page.tsx

@@ -0,0 +1,63 @@
+import { TraceList } from "@/components/trace-list";
+
+interface TracesResponse {
+  traces: Array<{
+    id: string;
+    name: string;
+    status: "RUNNING" | "COMPLETED" | "ERROR";
+    startedAt: string;
+    endedAt: string | null;
+    durationMs: number | null;
+    tags: string[];
+    metadata: Record<string, unknown>;
+    _count: {
+      decisionPoints: number;
+      spans: number;
+      events: number;
+    };
+  }>;
+  total: number;
+  page: number;
+  limit: number;
+  totalPages: number;
+}
+
+async function getTraces(
+  limit = 50,
+  page = 1
+): Promise<TracesResponse> {
+  try {
+    const res = await fetch(
+      `http://localhost:3000/api/traces?limit=${limit}&page=${page}`,
+      { cache: "no-store" }
+    );
+
+    if (!res.ok) {
+      throw new Error(`Failed to fetch traces: ${res.status}`);
+    }
+
+    return res.json();
+  } catch (error) {
+    console.error("Error fetching traces:", error);
+    return {
+      traces: [],
+      total: 0,
+      page: 1,
+      limit,
+      totalPages: 0,
+    };
+  }
+}
+
+export default async function DashboardPage() {
+  const data = await getTraces(50, 1);
+
+  return (
+    <TraceList
+      initialTraces={data.traces}
+      initialTotal={data.total}
+      initialTotalPages={data.totalPages}
+      initialPage={data.page}
+    />
+  );
+}