feat(user-management): Enhance admin management and role handling

- Add `CountEnabledAdminsExcluding` function to count enabled admins excluding a specific user.
- Implement `CountUsersByRoleAndEnabledExclude` in `internal/db/user.go` to support exclusion logic.
- Refactor role handling with switch-case for better readability in `server/handles/role.go`.
- Ensure at least one enabled admin remains when disabling an admin in `server/handles/user.go`.
- Maintain guest role name consistency when updating roles in `internal/op/role.go`.

internal/db/user.go

@@ -2,6 +2,7 @@ package db
 
 import (
 	"encoding/base64"
+	"fmt"
 	"github.com/alist-org/alist/v3/internal/model"
 	"github.com/alist-org/alist/v3/pkg/utils"
 	"github.com/go-webauthn/webauthn/webauthn"
@@ -140,3 +141,13 @@ func UpdateUserBasePathPrefix(oldPath, newPath string) ([]string, error) {
 
 	return modifiedUsernames, nil
 }
+
+func CountUsersByRoleAndEnabledExclude(roleID uint, excludeUserID uint) (int64, error) {
+	var count int64
+	jsonValue := fmt.Sprintf("[%d]", roleID)
+	err := db.Model(&model.User{}).
+		Where("disabled = ? AND id != ?", false, excludeUserID).
+		Where("JSON_CONTAINS(role, ?)", jsonValue).
+		Count(&count).Error
+	return count, err
+}

internal/op/role.go

@@ -96,8 +96,12 @@ func UpdateRole(r *model.Role) error {
 	if err != nil {
 		return err
 	}
-	if old.Name == "admin" || old.Name == "guest" {
+	switch old.Name {
+	case "admin":
 		return errs.ErrChangeDefaultRole
+
+	case "guest":
+		r.Name = "guest"
 	}
 	for i := range r.PermissionScopes {
 		r.PermissionScopes[i].Path = utils.FixAndCleanPath(r.PermissionScopes[i].Path)

internal/op/user.go

@@ -136,3 +136,11 @@ func DelUserCache(username string) error {
 	userCache.Del(username)
 	return nil
 }
+
+func CountEnabledAdminsExcluding(userID uint) (int64, error) {
+	adminRole, err := GetRoleByName("admin")
+	if err != nil {
+		return 0, err
+	}
+	return db.CountUsersByRoleAndEnabledExclude(adminRole.ID, userID)
+}