refactor: Download API

Now the download APIs are moved into a new file

backend/src/utils/flow.js

@@ -0,0 +1,15 @@
+import { HTTP } from '@/vendor/open-api';
+
+export async function getFlowHeaders(url) {
+    const http = HTTP();
+    const { headers } = await http.get({
+        url,
+        headers: {
+            'User-Agent': 'Quantumult%20X/1.0.30 (iPhone14,2; iOS 15.6)',
+        },
+    });
+    const subkey = Object.keys(headers).filter((k) =>
+        /SUBSCRIPTION-USERINFO/i.test(k),
+    )[0];
+    return headers[subkey];
+}