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/logger/logger.go

@@ -5,51 +5,29 @@ import (
 	"os"
 )
 
-var (
-	defaultLevel = slog.LevelInfo
-	logger       *slog.Logger
-)
+var currentLevel = slog.LevelInfo
 
 func init() {
 	opts := &slog.HandlerOptions{
-		Level: defaultLevel,
+		Level: currentLevel,
 	}
-	logger = slog.New(slog.NewJSONHandler(os.Stdout, opts))
+	logger := slog.New(slog.NewJSONHandler(os.Stdout, opts))
 	slog.SetDefault(logger)
 }
 
 func SetLevel(level slog.Level) {
+	currentLevel = level
 	opts := &slog.HandlerOptions{
-		Level: level,
+		Level: currentLevel,
 	}
-	logger = slog.New(slog.NewJSONHandler(os.Stdout, opts))
+	logger := slog.New(slog.NewJSONHandler(os.Stdout, opts))
 	slog.SetDefault(logger)
 }
 
 func SetTextHandler() {
 	opts := &slog.HandlerOptions{
-		Level: defaultLevel,
+		Level: currentLevel,
 	}
-	logger = slog.New(slog.NewTextHandler(os.Stdout, opts))
+	logger := slog.New(slog.NewTextHandler(os.Stdout, opts))
 	slog.SetDefault(logger)
 }
-
-func Debug(msg string, args ...any) {
-	slog.Debug(msg, args...)
-}
-
-func Info(msg string, args ...any) {
-	slog.Info(msg, args...)
-}
-
-func Warn(msg string, args ...any) {
-	slog.Warn(msg, args...)
-}
-
-func Error(msg string, args ...any) {
-	slog.Error(msg, args...)
-}
-
-func With(args ...any) *slog.Logger {
-	return slog.With(args...)
-}