feat: implement backend API clients for multi-marketplace integration and frontend service layer

frontend/js/api.js

@@ -52,3 +52,18 @@ export async function listFiles(caseId) {
   const res = await fetch(`${API_BASE}/cases/${caseId}/files`);
   return await res.json();
 }
+
+export async function getConfiguredPlatforms() {
+  const res = await fetch(`${API_BASE}/credentials`);
+  const data = await res.json();
+  return data.configured_platforms || [];
+}
+
+export async function saveCredentials(platform, data) {
+  const res = await fetch(`${API_BASE}/credentials/${platform}`, {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(data),
+  });
+  return await res.json();
+}