[native] Add loading button state to CreateSIWEBackupMessageBase

Summary:
Noticed that when the user presses the button to open `SIWEPanel`, we don't change the button to a loading state like we do for the `ConnectEthereum` screen [here](https://github.com/CommE2E/comm/blob/5b1ddea9fbe3a2d7b65e73193dfa16cda701c995/native/account/registration/connect-ethereum.react.js#L236-L239).

This diff adds the loading state to the button, which improves the responsiveness of the UX.

Test Plan: I tested in my local environment to make sure the button changed to a loading state

Reviewers: will, varun

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13804

native/account/registration/siwe-backup-message-creation.react.js

@@ -71,9 +71,11 @@ const CreateSIWEBackupMessageBase: React.ComponentType<CreateSIWEBackupMessageBa
       const newSignatureButtonText = onExistingWalletSignature
         ? 'Encrypt with new signature'
         : 'Encrypt with Ethereum signature';
-      const newSignatureButtonVariant = onExistingWalletSignature
+      const defaultNewSignatureButtonVariant = onExistingWalletSignature
         ? 'outline'
         : 'enabled';
+      const newSignatureButtonVariant =
+        panelState === 'opening' ? 'loading' : defaultNewSignatureButtonVariant;
 
       let useExistingSignatureButton;
       if (onExistingWalletSignature) {