feat(frontend): contract types + REST client (MSW-tested)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/sql.ts

@@ -0,0 +1,14 @@
+import { apiFetch } from "./client";
+import type { PreviewResult } from "./types";
+
+export const sqlPreview = (
+  id: string,
+  opts: { limit?: number; offset?: number } = {}
+) =>
+  apiFetch<PreviewResult>(`/sessions/${id}/sql/preview`, {
+    method: "POST",
+    body: JSON.stringify(opts),
+  });
+
+export const sqlExport = (id: string) =>
+  apiFetch<{ path: string }>(`/sessions/${id}/sql/export`, { method: "POST" });