[Frontend] Add Preferred Currency to User Settings

[FelipePSoares]

Extend the user settings UI to include an option for selecting PreferredCurrency.

• [x] Update the first-login page to include a dropdown for selecting PreferredCurrency.
• [ ] Update the user profile settings page to include a dropdown for selecting PreferredCurrency.
• [x] Save the selected currency by calling the backend API.
• [ ] Ensure the frontend defaults to the user’s preferred currency for all relevant displays.