fix(ui): improve breadcrumb current page semantics

[wyf027]

Summary

• Mark the current breadcrumb item with aria-current="page", defaulting to the last item unless an item sets current explicitly.
• Hide breadcrumb separators from assistive technology with aria-hidden.
• Render static breadcrumb items as text instead of inert buttons, while preserving links and onClick buttons.
• Add Breadcrumb tests for landmark/list structure, current page semantics, explicit current override, hidden separators, and clickable non-link items.
• Update Breadcrumb docs for accessibility behavior, separator: ReactNode, and BreadcrumbItem.current.

Gap Notes

• WAI-ARIA Breadcrumb pattern expects a labelled navigation landmark and aria-current="page" on the current-page link: https://www.w3.org/WAI/ARIA/apg/patterns/breadcrumb/
• MDN documents aria-current="page" for current breadcrumb links and notes that a non-link current page item can also represent the current location: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Reference/Attributes/aria-current
• Element Plus Breadcrumb describes the component as current page location navigation and exposes separator customization: https://element-plus.org/en-US/component/breadcrumb

Validation

• npm ci
• npx prettier --write packages/ui/src/components/navigation/Breadcrumb/Breadcrumb.tsx packages/ui/src/components/navigation/Breadcrumb/Breadcrumb.test.tsx docs/components/breadcrumb.md
• npm run lint:ui
• npm run typecheck:ui
• npm run test:ui (24 files, 75 tests)
• npm run build:ui
• npm run build

Notes

• npm ci still reports existing Node 23 engine warnings and existing npm audit findings (1 low, 4 moderate, 2 high).
• npm run build still reports existing Browserslist and VitePress chunk-size warnings.

[ecc-tools]

ECC bundle files are already tracked in this repository. Skipping generation of another bundle PR.