Add post-installation redirect based on admin account status (#34493)

This PR adds a feature to direct users to appropriate pages after system
installation:
- If no admin credentials were provided during installation, redirect to
the registration page with a prominent notice about creating the first
administrative account
- If admin credentials were already set, redirect directly to the login
page


![4d396ad132d9b57fc4f45a62117177f1](https://github.com/user-attachments/assets/3a5d8700-9194-4d3b-a862-e64c8c347932)

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/user/user.go

@@ -831,6 +831,20 @@ type CountUserFilter struct {
 	IsActive       optional.Option[bool]
 }
 
+// HasUsers checks whether there are any users in the database, or only one user exists.
+func HasUsers(ctx context.Context) (ret struct {
+	HasAnyUser, HasOnlyOneUser bool
+}, err error,
+) {
+	res, err := db.GetEngine(ctx).Table(&User{}).Cols("id").Limit(2).Query()
+	if err != nil {
+		return ret, fmt.Errorf("error checking user existence: %w", err)
+	}
+	ret.HasAnyUser = len(res) != 0
+	ret.HasOnlyOneUser = len(res) == 1
+	return ret, nil
+}
+
 // CountUsers returns number of users.
 func CountUsers(ctx context.Context, opts *CountUserFilter) int64 {
 	return countUsers(ctx, opts)