feat: add NavigationControls with progress indicator

[erikhfj]

Summary

Closes #13 (taking over from evenkkl, who is sick — get well soon!):

• NavigationControls: props-driven { currentIndex, totalCount, onPrevious, onNext }
• Position indicator 3 / 10 with aria-live=polite so screen readers announce position changes
• Both buttons disabled at the ends; aria-labels (Show previous book / Show next book); visible focus-visible outline (graded a11y)
• Plain CSS imported by the component, consistent with the app look

Tests (4 new, 23 total green)

• Position rendering (3 / 10, 10 / 10 at the end)
• Click → correct handler called
• Boundary disabling: click on disabled button → handler not called (verified with rerender to both ends)
• Snapshot

Checklist

• npm run lint — 0 warnings
• npm test — 23 passed
• npm run build — passes
• npm run format:check — passes

Note for #12/#22: once this merges, the App wiring adds useState for the index and renders NavigationControls + BookJumpList.

AI-assisted: implementation generated with AI assistance, reviewed and verified.

[rachelks]

Looks good! The implementation matches the acceptance criteria, and the tests cover the navigation and boundary cases well. Tested locally and everything passes.