feat(button): replace btnType with variant and color

[wangdicoder]

Summary

• replace the Button, Button.Group, and SplitButton btnType API with variant and color
• rebuild button styles and button tokens around variant/color combinations
• migrate docs, demos, snapshots, and downstream usages to the new API

Breaking changes

• remove btnType and ButtonType
• add variant: solid | outline | ghost | link
• add color: default | primary | info | success | warning | danger
• rename button design tokens to variant/color-based keys such as button.solid.primary.bg

Validation

• pnpm --filter @tiny-design/react exec jest button split-button transfer
• pnpm --filter @tiny-design/react build
• pnpm --filter @tiny-design/tokens exec node scripts/build.js
• pnpm --filter @tiny-design/docs build

Notes

• includes a major changeset for @tiny-design/react and @tiny-design/tokens
• keeps the recent loading, icon-position, shape, and group inheritance improvements under the new API model