Implement Google OAuth flow and enhance login functionality

- Updated the Google Sign-In integration to utilize the new OAuth callback mechanism.
- Added a redirect flow for Google authentication, improving user experience.
- Enhanced error handling and user feedback during the login process.
- Removed hardcoded Google client ID in favor of environment variable usage.
- Introduced a new component for handling OAuth callbacks and updated the App component to manage authentication states accordingly.
- Improved API route for processing OAuth callbacks, including token exchange and user verification.

webpack.config.js

@@ -44,7 +44,7 @@ module.exports = {
     new HtmlWebpackPlugin({
       template: './client/public/index.html',
       templateParameters: {
-        REACT_APP_GOOGLE_CLIENT_ID: process.env.GOOGLE_CLIENT_ID || 'your_google_client_id_here',
+        REACT_APP_GOOGLE_CLIENT_ID: process.env.GOOGLE_CLIENT_ID,
       },
     }),
     new webpack.DefinePlugin({