feat(schedule): add course table screens and navigation

Add complete schedule functionality including:
- Schedule screen with weekly course table view
- Course detail screen with transparent modal presentation
- New ScheduleStack navigator integrated into main tab bar
- Schedule service for API interactions
- Type definitions for course entities

Also includes bug fixes for group invite/request handlers
to include required groupId parameter.

internal/repository/schedule_repo.go

@@ -0,0 +1,66 @@
+package repository
+
+import (
+	"carrot_bbs/internal/model"
+
+	"gorm.io/gorm"
+)
+
+type ScheduleRepository interface {
+	ListByUserID(userID string) ([]*model.ScheduleCourse, error)
+	GetByID(id string) (*model.ScheduleCourse, error)
+	Create(course *model.ScheduleCourse) error
+	Update(course *model.ScheduleCourse) error
+	DeleteByID(id string) error
+	ExistsColorByUser(userID, color, excludeID string) (bool, error)
+}
+
+type scheduleRepository struct {
+	db *gorm.DB
+}
+
+func NewScheduleRepository(db *gorm.DB) ScheduleRepository {
+	return &scheduleRepository{db: db}
+}
+
+func (r *scheduleRepository) ListByUserID(userID string) ([]*model.ScheduleCourse, error) {
+	var courses []*model.ScheduleCourse
+	err := r.db.
+		Where("user_id = ?", userID).
+		Order("day_of_week ASC, start_section ASC, created_at ASC").
+		Find(&courses).Error
+	return courses, err
+}
+
+func (r *scheduleRepository) Create(course *model.ScheduleCourse) error {
+	return r.db.Create(course).Error
+}
+
+func (r *scheduleRepository) GetByID(id string) (*model.ScheduleCourse, error) {
+	var course model.ScheduleCourse
+	if err := r.db.Where("id = ?", id).First(&course).Error; err != nil {
+		return nil, err
+	}
+	return &course, nil
+}
+
+func (r *scheduleRepository) Update(course *model.ScheduleCourse) error {
+	return r.db.Save(course).Error
+}
+
+func (r *scheduleRepository) DeleteByID(id string) error {
+	return r.db.Delete(&model.ScheduleCourse{}, "id = ?", id).Error
+}
+
+func (r *scheduleRepository) ExistsColorByUser(userID, color, excludeID string) (bool, error) {
+	var count int64
+	query := r.db.Model(&model.ScheduleCourse{}).
+		Where("user_id = ? AND LOWER(color) = LOWER(?)", userID, color)
+	if excludeID != "" {
+		query = query.Where("id <> ?", excludeID)
+	}
+	if err := query.Count(&count).Error; err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}