feat: add JSON schema widget support with checkbox and radio inputs

Summary

• Add widget.ts to @visual-json/core with logic to resolve widget types from JSON Schema (enum → select/radio, boolean → checkbox)
• Add CheckboxInput and RadioInput components to @visual-json/react with full test coverage
• Update form-view.tsx to render the appropriate widget based on schema
• Refactor editor.tsx in the React example: extract static samples data to samples.ts for readability

Test plan

• pnpm test — 203 tests pass
• pnpm type-check — 0 errors
• pnpm lint — 0 warnings
• pnpm build — all 11 packages build successfully
• Manual: open React example, select "Schema widgets" sample — verify checkbox and radio/select widgets render correctly

Closes #25