Define border-radius design tokens

What does this MR do?

Adds border.radius design tokens.

• Adds constant border radius design tokens
• Updates Tailwind defaults with constant border radius design tokens
• Adds semantic border radius design for action and control
• Adds control border radius to form inputs and select
• Adds context border radius design tokens for components

Note: fixed rem/px calc issue in product here gitlab-org/gitlab!204134 (merged)

Discussions

• !5049 (comment 2736741838)
• !5049 (comment 2736698619)
• !5049 (comment 2736696880)
• #2986 (comment 2740591099)
• !4888 (comment 2708657265)

Screenshots or screen recordings

CleanShot_2025-09-10_at_17.01.39

Integrations

• GitLab: gitlab-org/gitlab!205472 (closed)

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

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Conformity

• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
• Relevant label(s) are applied to the MR.
• The MR is added to a milestone.
• Added the ~"component:*" label(s) if applicable.
• A changeset is added when this MR will do a patch, minor, or major update. More information can be found in doc/publishing-packages.md.

Components

• GitLab UI's contributing guidelines.
• If the MR changes a component's API, integration MR(s) have been opened (see integrations above).

Documentation

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Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

Related to #2957 (closed)