feat(tag): add variant prop

[wangdicoder]

Summary

• Added variant prop to Tag component with four styles: filled (default), soft, solid, and outlined
• Updated SCSS styles to support all variant combinations for preset colors and status colors
• Added TagVariant type export
• Updated documentation with new Variant demo section

Release

• Bump type: minor
• Affected packages: @tiny-design/react

Test plan

• Run pnpm --filter @tiny-design/react test -- --testPathPattern=tag to verify tests pass
• Visually verify all four variant styles in the docs dev server
• Test that variant works correctly with custom hex colors

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