refactor: modularize API client by splitting resources into individual files and standardizing HTTP request handling.

apps/frontend/src/lib/api/resources/plans.ts

@@ -0,0 +1,7 @@
+import type { PlanSummary } from '@repo/api-contract';
+import { http } from '../http';
+
+export async function list() {
+  const response = await http.get<PlanSummary[]>('/api/plans');
+  return response.data;
+}