Refactor the gRPC client and task handling logic to improve modularity,
error handling, and observability.
- Split `grpc/handler.go` into specialized handler files (login,
classroom, exams, grades, schedule) to reduce complexity.
- Replace standard `log` with `log/slog` throughout the project for
structured logging.
- Refactor `grpc/client.go` to use a thread-safe connection management
pattern with `sync.RWMutex`.
- Remove the legacy `service` package and HTTP server implementation,
transitioning the application to a pure gRPC runner mode.
- Clean up `config` and `pkg/errors` to remove unused utilities and
simplify the API.
- Improve error reporting in parsers and HTTP clients by checking
response status codes.
