feat(design-system): expand the foundational UI component library

[kev1n77]

Summary

• Add seven reusable UI primitives and compositions:
  • ActionItem
  • Field
  • IconButton
  • Input
  • KeyHint
  • PageHeader
  • SearchField
• Add public exports, component metadata, registry entries, semantic styles, and focused contract tests for each component.
• Expand Button with primary and text presentation variants while preserving the existing default behavior.
• Extend Design Lab with localized documentation, state previews, inspector controls, and usage examples for the new components.
• Fix preview matrix layouts so state samples remain horizontal and wide ActionItem compositions do not overlap.

Fixes #

Type and Areas

Type:

Feature / UI/UX / regression fix / test

Areas:

Design system UI package, Design Lab, component registry, component documentation and previews

Motivation / Impact

This expands the new component library with reusable action, form, keyboard-hint, search, and page-heading contracts. It provides a stronger foundation for gradually replacing legacy or locally implemented UI with consistent design-system components.

The new APIs keep semantics, presentation, state, and composition independently configurable. Existing Button behavior remains compatible because its default variant is unchanged.

There is no direct end-user change outside Design Lab in this PR because existing application call sites are not migrated yet. Developers can begin adopting the new components incrementally.

Verification

• pnpm --filter @bitfun/design-lab test
  • Passed all Design Lab tests and TypeScript checks.
• pnpm run design-system:check
  • Built design tokens, theme packages, the UI package, and Design Lab successfully.
  • Passed token-engine, design-token, theme, UI component, package-contract, and Design Lab test suites.
• Manually checked the Design Lab state matrices:
  • Preview states render horizontally.
  • ActionItem preserves spacing between its leading content, label, shortcut, and trailing actions.
  • Narrow preview areas retain complete component anatomy through horizontal scrolling.

Reviewer Notes

• This PR only establishes and documents design-system contracts; it does not migrate application call sites.
• The added components are registered as stable public components and exported through @bitfun/ui.
• Design Lab sizing rules are preview-only and do not alter component behavior in consuming applications.
• The Button variant union is additive; existing outline and fill usages and the default outline behavior are preserved.
• Rollback is isolated to the design-system package and Design Lab changes.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.