feat: add BookJumpList component (#14)

[rachelks]

Summary

Closes #14.

• Adds a props-driven BookJumpList component
• Uses a native <select> wrapped in an accessible <label>
• Uses currentIndex to reflect the currently selected book
• Calls onSelect with the selected zero-based index
• Adds responsive styling consistent with the existing app
• Adds component tests using inline fixture titles only
• Adds snapshot coverage
• Does not fetch data or access the network

Validation

• npm run format:check
• npm run lint
• npm test
• npm run build

[robertky]

Looks good to me. The component is accessible, well tested, and matches the requirements in #14.