chore: Add themeable tokens for button

[at-susie]

Description

This PR adds independently themeable design tokens for the button component. Specifically it introduces dedicated color tokens for the link button variant to achieve parity with normal and primary variants, and adds a new spaceButtonVertical spacing token so button padding is themeable in both directions. No visual changes — all new tokens default to the previous values.

Related links, issue #, if available: n/a

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.45%. Comparing base (a6cdcd0) to head (1fafc84).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4395   +/-   ##
=======================================
  Coverage   97.45%   97.45%           
=======================================
  Files         909      909           
  Lines       26637    26637           
  Branches     9614     9615    +1     
=======================================
  Hits        25958    25958           
+ Misses        673      636   -37     
- Partials        6       43   +37     

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