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>

routers/web/auth/auth.go

@@ -421,9 +421,11 @@ func SignOut(ctx *context.Context) {
 // SignUp render the register page
 func SignUp(ctx *context.Context) {
 	ctx.Data["Title"] = ctx.Tr("sign_up")
-
 	ctx.Data["SignUpLink"] = setting.AppSubURL + "/user/sign_up"
 
+	hasUsers, _ := user_model.HasUsers(ctx)
+	ctx.Data["IsFirstTimeRegistration"] = !hasUsers.HasAnyUser
+
 	oauth2Providers, err := oauth2.GetOAuth2Providers(ctx, optional.Some(true))
 	if err != nil {
 		ctx.ServerError("UserSignUp", err)
@@ -610,7 +612,13 @@ func createUserInContext(ctx *context.Context, tpl templates.TplName, form any,
 // sends a confirmation email if required.
 func handleUserCreated(ctx *context.Context, u *user_model.User, gothUser *goth.User) (ok bool) {
 	// Auto-set admin for the only user.
-	if user_model.CountUsers(ctx, nil) == 1 {
+	hasUsers, err := user_model.HasUsers(ctx)
+	if err != nil {
+		ctx.ServerError("HasUsers", err)
+		return false
+	}
+	if hasUsers.HasOnlyOneUser {
+		// the only user is the one just created, will set it as admin
 		opts := &user_service.UpdateOptions{
 			IsActive:     optional.Some(true),
 			IsAdmin:      user_service.UpdateOptionFieldFromValue(true),