feat: implement Nest API client with bearer token authentication
Implemented a central API client for communication with the NestJS backend.
The client authenticates via the /api/auth/login endpoint and retrieves a JWT bearer token.
This token is automatically attached to all subsequent API requests using the Authorization header.
This allows the frontend to access protected backend endpoints without handling authentication logic in individual components.
Testing: The connection was tested by requesting player data from the backend API and verifying that the response was returned correctly.