More Sign up and Login UI Templates

## Messaging

Authgear provides prebuilt signup and login pages that are designed for frictionless user experience. Save time to build sign up pages that look great and achieve high conversion rate.

## Objectives

To test different signup page designs for us. And to offer more customization options in terms of UI design to our users.

## User Stories/Requirements

- As an app developer, I can choose among 3 signup/login page designs from the portal, and customize the colors and copywriting according to my needs.
- There should be 2 additional designs. The designs should accommodate all the login methods and interactions in AuthUI, except the `/settings` pages, which include forgot password, change password, OTP setup, logout, error page, verify email, etc. (Ref: List of UI templates - #1200 )
- The design should be responsive and works on desktop, tablet and mobile devices.

## User flow

1. Portal > UI Settings > Theme
2. Select Theme for signup and login page
3. Customization on color and copywriting
4. Live Preview
5. Press “Save” to change the settings and all end-users will see the new UI.

## Design reference

1. Design 1 - include a space for headline and benefit on the side. e.g. https://identity.getpostman.com/signup

2. Design 2 - a clean and modern design e.g. https://admin.typeform.com/signup

## Appetite

2 weeks

## Implementation Note

The implementation should fix #1689 . I.e. the email and passwords are filled together with 1 click when using password manager apps.

## Todo

- [x] #1900
- [ ] #1901

Start at Portal > UI Settings > Theme and review the existing AuthUI templates, using the referenced template list in #1200 and the designs linked in the issue as context. Done means adding two selectable, responsive signup/login designs with color and copy customization, live preview, save behavior, support for AuthUI login methods, and the password-manager behavior described for #1689.

Written by the indexing model from the issue text.