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_helpers.go

@@ -0,0 +1,108 @@
+package grpc
+
+import (
+	"context"
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"net/http/cookiejar"
+	"net/url"
+	"time"
+
+	"schedule_converter/auth"
+	"schedule_converter/captcha"
+	"schedule_converter/client"
+	"schedule_converter/config"
+	"schedule_converter/models"
+	"schedule_converter/parser"
+	apperr "schedule_converter/pkg/errors"
+)
+
+const (
+	maxLoginRetries    = 3
+	loginRetryInterval = 3 * time.Second
+)
+
+func createHTTPClient() (*http.Client, error) {
+	jar, err := cookiejar.New(nil)
+	if err != nil {
+		return nil, apperr.New("create_jar", err, "创建 cookie jar 失败")
+	}
+	return &http.Client{Jar: jar}, nil
+}
+
+func setTWFIDCookie(httpClient *http.Client, baseURL, twfid string) {
+	cfg := config.Get()
+	if twfid == "" {
+		twfid = cfg.TWFID
+	}
+	u, err := url.Parse(baseURL)
+	if err != nil {
+		slog.Warn("解析 baseURL 失败，无法设置 TWFID cookie", "base_url", baseURL, "error", err)
+		return
+	}
+	httpClient.Jar.SetCookies(u, []*http.Cookie{
+		{Name: "TWFID", Value: twfid},
+	})
+}
+
+func executeLogin(ctx context.Context, httpClient *http.Client, username, password string) error {
+	log := slog.With("username", username)
+
+	if err := client.GetInitialCookiesWithClient(httpClient); err != nil {
+		return apperr.New("get_cookies", err, "获取初始 Cookie 失败")
+	}
+
+	for attempt := 1; attempt <= maxLoginRetries; attempt++ {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+		}
+
+		log.Info("尝试登录", "attempt", attempt, "max_retries", maxLoginRetries)
+
+		captchaCode, err := captcha.GetAndRecognize(httpClient)
+		if err != nil {
+			log.Warn("验证码获取失败", "error", err, "attempt", attempt)
+			time.Sleep(loginRetryInterval)
+			continue
+		}
+
+		success, err := auth.LoginWithClient(httpClient, captchaCode, username, password)
+		if err != nil {
+			log.Warn("登录请求失败", "error", err, "attempt", attempt)
+			time.Sleep(loginRetryInterval)
+			continue
+		}
+
+		if success {
+			return nil
+		}
+
+		log.Warn("登录验证失败", "attempt", attempt)
+		time.Sleep(loginRetryInterval)
+	}
+
+	return apperr.ErrLoginFailed
+}
+
+func executeLoginAndFetchSchedule(ctx context.Context, httpClient *http.Client, username, password, semester string) ([]models.Course, error) {
+	if err := executeLogin(ctx, httpClient, username, password); err != nil {
+		return nil, err
+	}
+
+	cfg := config.Get()
+	if semester == "" {
+		semester = cfg.Semester
+	}
+
+	return parser.FetchScheduleWithClient(httpClient, semester)
+}
+
+func unmarshalPayload(payload []byte, v any) error {
+	if err := json.Unmarshal(payload, v); err != nil {
+		return apperr.New("parse_payload", err, "解析任务载荷失败")
+	}
+	return nil
+}