fix(button): align list-variant label with icon line-box

[kelsos]

Summary

• Tighten the variant=\"list\" button label to leading-[1.125rem] so its line-box matches the md icon (--rui-icon-size: 1.125rem). Without this, the 20px label line-box (from leading-5 on the root) is flex-centered against the 18px icon box and the baseline-aligned glyphs visually drift above the icon's optical center — most readable in tight list rows like the rotki UserDropdown menu.
• Add a list-variant section to the example app (size grid with prepend-icon rows mirroring the menu pattern).
• Add an e2e assertion that the label line-height collapses to 18px and that the label/icon centers line up.

Closes #515

Test plan

• pnpm -w run typecheck
• pnpm -w run lint
• pnpm test:run RuiButton.spec.ts — 21 pass
• pnpm test:e2e button.spec.ts — 2 pass (existing + new list-variant alignment test)
• Visual check in the example app (/buttons) — list-variant section, every size, label sits centered with the prepend icon

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.87%. Comparing base (2c15f1c) to head (3b153a3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #520   +/-   ##
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  Coverage   84.87%   84.87%           
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  Files         143      143           
  Lines        5209     5209           
  Branches     1553     1553           
=======================================
  Hits         4421     4421           
  Misses        788      788           

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