refactor: replace skill code with skill ID across the application

- Updated skill handling to use skill IDs instead of skill codes in various components, services, and models.
- Modified tests to reflect changes in skill identification.
- Removed references to skill codes in favor of skill IDs for consistency and clarity.
- Updated localization files to remove skill code references and adjust error messages accordingly.

internal/repositories/skill_definition_repository.go

@@ -101,10 +101,6 @@ func (r *skillDefinitionRepository) Delete(db *gorm.DB, id int64) {
 	db.Delete(&models.SkillDefinition{}, "id = ?", id)
 }
 
-func (r *skillDefinitionRepository) GetByCode(db *gorm.DB, code string) *models.SkillDefinition {
-	return r.FindOne(db, sqls.NewCnd().Where("code = ?", code))
-}
-
 func (r *skillDefinitionRepository) GetByIDs(db *gorm.DB, ids []int64) map[int64]models.SkillDefinition {
 	if len(ids) == 0 {
 		return nil