refactor(grpc): restructure task handling and migrate to slog

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.

pkg/errors/errors.go

@@ -5,16 +5,7 @@ import (
 	"fmt"
 )
 
-var (
-	ErrInvalidConfig     = errors.New("invalid configuration")
-	ErrLoginFailed       = errors.New("login failed")
-	ErrCaptchaFailed     = errors.New("captcha recognition failed")
-	ErrScheduleFailed    = errors.New("failed to fetch schedule")
-	ErrRecognitionFailed = errors.New("schedule recognition failed")
-	ErrUnauthorized      = errors.New("unauthorized access")
-	ErrTimeout           = errors.New("operation timeout")
-	ErrConnectionFailed  = errors.New("connection failed")
-)
+var ErrLoginFailed = errors.New("login failed")
 
 type ScheduleError struct {
 	Op  string
@@ -40,19 +31,3 @@ func New(op string, err error, msg string) error {
 		Msg: msg,
 	}
 }
-
-func Wrap(err error, msg string) error {
-	return fmt.Errorf("%s: %w", msg, err)
-}
-
-func Join(errs ...error) error {
-	return errors.Join(errs...)
-}
-
-func Is(err, target error) bool {
-	return errors.Is(err, target)
-}
-
-func As(err error, target any) bool {
-	return errors.As(err, target)
-}