feat(a11y): add focus trap to Modal and manage focus on route changes (#229)

[solomonokeke012-bot]

Summary

Closes #229

Implements proper focus management per the accessibility issue spec.

Changes

• src/components/ui/Modal.tsx (new) — Reusable accessible modal with:

  • Focus trap via existing useFocusTrap hook (focus returns to trigger on close)
  • role="dialog", aria-modal, aria-labelledby with stable useId()
  • Escape key and backdrop-click to close
  • Body scroll lock while open
  • Screen reader announcement on open

• src/hooks/useAccessibility.tsx — Added useFocusOnRouteChange(pathname) hook that moves focus to the <main> landmark on every route change

• src/components/accessibility/RouteChangeAnnouncer.tsx (new) — Thin client component that reads usePathname() and calls useFocusOnRouteChange; renders nothing

• src/providers/RootProviders.tsx — Mounts <RouteChangeAnnouncer /> inside <AccessibilityProvider> for app-wide route focus management

• src/components/index.ts — Exports Modal from the component barrel

Testing

• Open a modal → focus moves inside; Tab cycles within; Escape/backdrop closes and focus returns to trigger
• Navigate between routes → focus moves to <main> content area

Acceptance Criteria

• Focus trap in modals
• Focus returned to trigger on close
• Focus managed on route changes

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