add: background.js for automatic code input

public/scripts/background.js

@@ -0,0 +1,23 @@
+import * as OTPAuth from 'otpauth';
+
+// eslint-disable-next-line no-undef
+const api = typeof browser !== "undefined" ? browser : chrome;
+
+api.runtime.onMessage.addListener((request, _sender, sendResponse) => {
+  if (request.action === "GENERATE_TOKEN") {
+    api.storage.local.get(['shasecret'], (data) => {
+      if (data.shasecret) {
+        const totp = new OTPAuth.TOTP({
+          issuer: 'US',
+          label: '2FA',
+          algorithm: 'SHA1',
+          digits: 6,
+          period: 30,
+          secret: OTPAuth.Secret.fromBase32(data.shasecret.trim().toUpperCase()),
+        });
+        sendResponse({ token: totp.generate() });
+      }
+    });
+    return true;
+  }
+});