feat: 新增分类服务层，实现分类的创建、查询及层级管理功能。

internal/service/category_service.go

@@ -115,6 +115,18 @@ func (s *CategoryService) GetAllCategories(userID uint) ([]models.Category, erro
 	if err != nil {
 		return nil, fmt.Errorf("failed to get categories: %w", err)
 	}
+
+	// Auto-seed if empty
+	if len(categories) == 0 {
+		if err := s.initDefaultCategories(userID); err != nil {
+			// Log error but return empty list to avoid crashing
+			fmt.Printf("Failed to seed default categories for user %d: %v\n", userID, err)
+			return []models.Category{}, nil
+		}
+		// Fetch again after seeding
+		return s.repo.GetAll(userID)
+	}
+
 	return categories, nil
 }
 
@@ -124,6 +136,12 @@ func (s *CategoryService) GetCategoriesByType(userID uint, categoryType models.C
 	if err != nil {
 		return nil, fmt.Errorf("failed to get categories by type: %w", err)
 	}
+
+	// We don't auto-seed here because the user might just have no categories of this specific type
+	// but might have categories of the other type.
+	// But if they have absolutely no categories, GetAllCategories would have handled it.
+	// We can trust GetAllCategories or just leave this as is.
+
 	return categories, nil
 }
 
@@ -134,6 +152,16 @@ func (s *CategoryService) GetCategoryTree(userID uint) ([]models.Category, error
 	if err != nil {
 		return nil, fmt.Errorf("failed to get category tree: %w", err)
 	}
+
+	// Auto-seed if empty
+	if len(categories) == 0 {
+		if err := s.initDefaultCategories(userID); err != nil {
+			fmt.Printf("Failed to seed default categories for user %d: %v\n", userID, err)
+			return []models.Category{}, nil
+		}
+		return s.repo.GetAllWithChildren(userID)
+	}
+
 	return categories, nil
 }
 
@@ -311,3 +339,133 @@ func (s *CategoryService) GetCategoryPath(userID, id uint) ([]models.Category, e
 
 	return path, nil
 }
+
+// initDefaultCategories seeds default categories for a user
+func (s *CategoryService) initDefaultCategories(userID uint) error {
+	defaults := []struct {
+		Name      string
+		Type      models.CategoryType
+		Icon      string
+		SortOrder int
+		Children  []struct {
+			Name      string
+			Type      models.CategoryType
+			Icon      string
+			SortOrder int
+		}
+	}{
+		// Expenses
+		{
+			Name: "餐饮", Type: models.CategoryTypeExpense, Icon: "restaurant", SortOrder: 1,
+			Children: []struct {
+				Name      string
+				Type      models.CategoryType
+				Icon      string
+				SortOrder int
+			}{
+				{Name: "早餐", Type: models.CategoryTypeExpense, Icon: "breakfast_dining", SortOrder: 1},
+				{Name: "午餐", Type: models.CategoryTypeExpense, Icon: "lunch_dining", SortOrder: 2},
+				{Name: "晚餐", Type: models.CategoryTypeExpense, Icon: "dinner_dining", SortOrder: 3},
+				{Name: "零食", Type: models.CategoryTypeExpense, Icon: "icecream", SortOrder: 4},
+				{Name: "饮料", Type: models.CategoryTypeExpense, Icon: "local_cafe", SortOrder: 5},
+			},
+		},
+		{
+			Name: "交通", Type: models.CategoryTypeExpense, Icon: "directions_bus", SortOrder: 2,
+			Children: []struct {
+				Name      string
+				Type      models.CategoryType
+				Icon      string
+				SortOrder int
+			}{
+				{Name: "地铁", Type: models.CategoryTypeExpense, Icon: "subway", SortOrder: 1},
+				{Name: "公交", Type: models.CategoryTypeExpense, Icon: "directions_bus", SortOrder: 2},
+				{Name: "打车", Type: models.CategoryTypeExpense, Icon: "local_taxi", SortOrder: 3},
+				{Name: "加油", Type: models.CategoryTypeExpense, Icon: "local_gas_station", SortOrder: 4},
+			},
+		},
+		{
+			Name: "购物", Type: models.CategoryTypeExpense, Icon: "shopping_bag", SortOrder: 3,
+			Children: []struct {
+				Name      string
+				Type      models.CategoryType
+				Icon      string
+				SortOrder int
+			}{
+				{Name: "服饰", Type: models.CategoryTypeExpense, Icon: "checkroom", SortOrder: 1},
+				{Name: "日用", Type: models.CategoryTypeExpense, Icon: "soap", SortOrder: 2},
+				{Name: "电子数码", Type: models.CategoryTypeExpense, Icon: "devices", SortOrder: 3},
+			},
+		},
+		{
+			Name: "居住", Type: models.CategoryTypeExpense, Icon: "home", SortOrder: 4,
+			Children: []struct {
+				Name      string
+				Type      models.CategoryType
+				Icon      string
+				SortOrder int
+			}{
+				{Name: "房租", Type: models.CategoryTypeExpense, Icon: "house", SortOrder: 1},
+				{Name: "水电煤", Type: models.CategoryTypeExpense, Icon: "lightbulb", SortOrder: 2},
+				{Name: "物业", Type: models.CategoryTypeExpense, Icon: "security", SortOrder: 3},
+			},
+		},
+		{
+			Name: "娱乐", Type: models.CategoryTypeExpense, Icon: "sports_esports", SortOrder: 5,
+			Children: []struct {
+				Name      string
+				Type      models.CategoryType
+				Icon      string
+				SortOrder int
+			}{
+				{Name: "游戏", Type: models.CategoryTypeExpense, Icon: "gamepad", SortOrder: 1},
+				{Name: "电影", Type: models.CategoryTypeExpense, Icon: "movie", SortOrder: 2},
+				{Name: "运动", Type: models.CategoryTypeExpense, Icon: "fitness_center", SortOrder: 3},
+			},
+		},
+		// Income
+		{
+			Name: "收入", Type: models.CategoryTypeIncome, Icon: "payments", SortOrder: 10,
+			Children: []struct {
+				Name      string
+				Type      models.CategoryType
+				Icon      string
+				SortOrder int
+			}{
+				{Name: "工资", Type: models.CategoryTypeIncome, Icon: "work", SortOrder: 1},
+				{Name: "奖金", Type: models.CategoryTypeIncome, Icon: "star", SortOrder: 2},
+				{Name: "兼职", Type: models.CategoryTypeIncome, Icon: "access_time", SortOrder: 3},
+				{Name: "理财", Type: models.CategoryTypeIncome, Icon: "trending_up", SortOrder: 4},
+			},
+		},
+	}
+
+	for _, cat := range defaults {
+		parent := &models.Category{
+			UserID:    userID,
+			Name:      cat.Name,
+			Type:      cat.Type,
+			Icon:      cat.Icon,
+			SortOrder: cat.SortOrder,
+		}
+		if err := s.repo.Create(parent); err != nil {
+			return err
+		}
+
+		for _, child := range cat.Children {
+			c := &models.Category{
+				UserID:    userID,
+				Name:      child.Name,
+				Type:      child.Type,
+				Icon:      child.Icon,
+				SortOrder: child.SortOrder,
+				ParentID:  &parent.ID,
+			}
+			if err := s.repo.Create(c); err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}