feat: share and manage ColorGradient presets

[otto-link]

Improve the ColorGradient widget to make user-created gradients reusable and easier to manage.

Requirements

• Persistent gradients: gradients created in one project should be available in other projects.

• Import / export: allow users to export gradients and import gradients shared by others.

• Favorites: allow users to pin/favorite frequently used gradients.

• Sorting: provide sorting options such as:

  • Name
  • Luminance
  • Other relevant properties

The goal is to make custom gradients reusable across projects and easy to organize, discover, and share.