Add Design page

[linear[bot]]

• Iframe: https://authgear.com/login?preview=inline
• Customisation input fields
  1. Organisation Name
  2. Logo
  3. Favicon
  4. Layout
  5. Card alignment
  6. Background
     1. color
     2. Image
  7. Button
     1. Primary button
     2. Primary button label
  8. Link color
  9. Input field
     1. border style
     2. border radius
  10. Hide authgear logo
• On value/configuration change
  • iframe.contentWindow.postMessage(__serialised_config__, authuiorigin)
• Save customisation config
  • Generate css, need to ask how existing code work

[linear[bot]]

DEV-1425 Add Design page