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/model/schedule_course.go

@@ -0,0 +1,35 @@
+package model
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+	"gorm.io/gorm"
+)
+
+// ScheduleCourse 用户课表课程
+type ScheduleCourse struct {
+	ID           string `json:"id" gorm:"type:varchar(36);primaryKey"`
+	UserID       string `json:"user_id" gorm:"type:varchar(36);index;not null"`
+	Name         string `json:"name" gorm:"type:varchar(120);not null"`
+	Teacher      string `json:"teacher" gorm:"type:varchar(80)"`
+	Location     string `json:"location" gorm:"type:varchar(120)"`
+	DayOfWeek    int    `json:"day_of_week" gorm:"index;not null"` // 0=周一, 6=周日
+	StartSection int    `json:"start_section" gorm:"not null"`
+	EndSection   int    `json:"end_section" gorm:"not null"`
+	Weeks        string `json:"weeks" gorm:"type:text;not null"` // JSON 数组字符串
+	Color        string `json:"color" gorm:"type:varchar(20)"`
+	CreatedAt    time.Time
+	UpdatedAt    time.Time
+}
+
+func (s *ScheduleCourse) BeforeCreate(tx *gorm.DB) error {
+	if s.ID == "" {
+		s.ID = uuid.New().String()
+	}
+	return nil
+}
+
+func (ScheduleCourse) TableName() string {
+	return "schedule_courses"
+}