refactor: Download API

Now the download APIs are moved into a new file

backend/src/utils/platform.js

@@ -0,0 +1,23 @@
+export function getPlatformFromHeaders(headers) {
+    const keys = Object.keys(headers);
+    let UA = '';
+    for (let k of keys) {
+        if (/USER-AGENT/i.test(k)) {
+            UA = headers[k];
+            break;
+        }
+    }
+    if (UA.indexOf('Quantumult%20X') !== -1) {
+        return 'QX';
+    } else if (UA.indexOf('Surge') !== -1) {
+        return 'Surge';
+    } else if (UA.indexOf('Decar') !== -1 || UA.indexOf('Loon') !== -1) {
+        return 'Loon';
+    } else if (UA.indexOf('Shadowrocket') !== -1) {
+        return 'Clash';
+    } else if (UA.indexOf('Stash') !== -1) {
+        return 'Stash';
+    } else {
+        return 'JSON';
+    }
+}