Refactor callback URL handling in injectVaranWallet function

- Changed callbackUrl in defaultInjectionConfig to an empty string.
- Updated injectVaranWallet to throw an error if callbackUrl is not provided in the config, ensuring proper error handling.

src/varan-wallet.ts

@@ -2,9 +2,7 @@ import type { WebApp, InjectionConfig } from './types';
 import { Wallet } from './wallet';
 
 const defaultInjectionConfig: Required<InjectionConfig> = {
-    get callbackUrl(): string {
-        throw new Error('Callback URL must be provided to injectVaranWallet');
-    },
+    callbackUrl: '',
     isDev: false,
     connectTimeout: 30_000,
     storage: {
@@ -20,6 +18,12 @@ const injectVaranWallet = (webApp: WebApp, config: InjectionConfig = defaultInje
 
   if (!isInTelegram) return;
 
+  const callbackUrl = config.callbackUrl || defaultInjectionConfig.callbackUrl;
+
+  if (!callbackUrl) {
+    throw new Error('Callback URL must be provided to injectVaranWallet');
+  }
+
   const wallet = new Wallet(webApp, { ...defaultInjectionConfig, ...config });
 
   window.injectedWeb3 = {