feat: ✨ restrict sd-brandshape to UI themes only

[coraliefeil]

User Story

As the Product Owner of the Solid Design System, I want the sd-brandshape component to be technically restricted to the ui-light and ui-dark theme only, so that it can't be used in non-Union Investment themes, since it is strictly brand-specific.

Suggested Solution

Since a technical or design-related restriction of usage ist not feasible, the component must at minimum be removed from visibility in documentation and design libraries (storybook, figma) to prevent unintended usage.

• DESIGN: In figma docs and library: add an info notification mentioning this component is exclusive to UI (eg. "Please notice that this component is only available for themes UI Light and UI Dark")
• CODE: In storybook docs: add an info notification mentioning this component is exclusive to UI (eg. "Please notice that this component is only available for themes UI Light and UI Dark")

Regarding the design & code component:

• DESIGN: Create color exception variables for branshape. for the "unavailable" themings we set the colors with 0% opacity and this way it is easy to see it is not visible/available.
• CODE: Create color exception variables for branshape. for the "unavailable" themings we set the colors with 0% opacity and this way it is easy to see it is not visible/available.
• DESIGN Figma component description: add a line mentioning this component is exclusive to UI (eg. "Please notice that this component is only available for themes UI Light and UI Dark")

Figma: https://www.figma.com/design/VTztxQ5pWG7ARg8hCX6PfR/branch/X1rnR0JYd2gbF7x25JD05H/Solid-DS-%E2%80%93-Component-Library?t=h2xikcaOUUKb8NS6-0

DoR

• Item has business value
• Item all subtasks have been estimated by the team
• Item is clear and well-defined
• Item dependencies have been identified

DoD

• All subtasks have been closed

[MartaPintoTeixeira]

https://www.figma.com/design/VTztxQ5pWG7ARg8hCX6PfR/branch/X1rnR0JYd2gbF7x25JD05H/Solid-DS-%E2%80%93-Component-Library?t=h2xikcaOUUKb8NS6-0
please review @coraliefeil
#2807
#2808