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.

grpc/handler_grades.go

@@ -0,0 +1,66 @@
+package grpc
+
+import (
+	"context"
+	"encoding/json"
+	"log/slog"
+
+	pb "schedule_converter/proto/runner"
+
+	"schedule_converter/config"
+	"schedule_converter/parser"
+	apperr "schedule_converter/pkg/errors"
+)
+
+func (h *TaskHandler) getGrades(ctx context.Context, payload []byte) ([]byte, error) {
+	var req pb.GetGradesPayload
+	if err := unmarshalPayload(payload, &req); err != nil {
+		return nil, err
+	}
+
+	log := slog.With("username", req.Username, "semester", req.Semester)
+	log.Info("获取成绩任务")
+
+	httpClient, err := createHTTPClient()
+	if err != nil {
+		return nil, err
+	}
+
+	cfg := config.Get()
+	setTWFIDCookie(httpClient, cfg.BaseURL, "")
+
+	if err := executeLogin(ctx, httpClient, req.Username, req.Password); err != nil {
+		return nil, err
+	}
+
+	var gpaSummary *pb.GpaSummary
+	summary, grades, err := parser.FetchGPA(httpClient)
+	if err != nil {
+		log.Warn("获取GPA页面失败，尝试成绩页面", "error", err)
+		grades, err = parser.FetchGrades(httpClient, parser.ScoreTypeFinal)
+		if err != nil {
+			return nil, apperr.New("fetch_grades", err, "获取成绩失败")
+		}
+	} else if summary != nil {
+		gpaSummary = &pb.GpaSummary{
+			AverageGpa:       summary.AverageGPA,
+			Rank:             summary.Rank,
+			ExamTotalGpa:     summary.ExamTotalGPA,
+			ExamTotalCredits: summary.ExamTotalCredits,
+			FailCredits:      summary.FailCredits,
+			Semesters:        summary.Semesters,
+		}
+	}
+
+	pbGrades := convertGrades(grades)
+
+	result := &pb.GradesResultData{
+		StudentId:  req.Username,
+		Semester:   req.Semester,
+		Grades:     pbGrades,
+		GpaSummary: gpaSummary,
+	}
+
+	log.Info("成绩获取成功", "grade_count", len(pbGrades))
+	return json.Marshal(result)
+}