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_login.go

@@ -0,0 +1,39 @@
+package grpc
+
+import (
+	"context"
+	"encoding/json"
+	"log/slog"
+
+	pb "schedule_converter/proto/runner"
+
+	"schedule_converter/config"
+)
+
+func (h *TaskHandler) login(ctx context.Context, payload []byte) ([]byte, error) {
+	var req pb.LoginPayload
+	if err := unmarshalPayload(payload, &req); err != nil {
+		return nil, err
+	}
+
+	log := slog.With("username", req.Username)
+	log.Info("登录任务")
+
+	httpClient, err := createHTTPClient()
+	if err != nil {
+		return nil, err
+	}
+
+	cfg := config.Get()
+	setTWFIDCookie(httpClient, cfg.BaseURL, req.Twfid)
+
+	if err := executeLogin(ctx, httpClient, req.Username, req.Password); err != nil {
+		return nil, err
+	}
+
+	log.Info("登录成功")
+	return json.Marshal(map[string]any{
+		"success": true,
+		"message": "登录成功",
+	})
+}