feat: Add carousel thumbnail navigation

[theMasudRana]

Summary

Adds slide thumbnail navigation to rtCarousel. This gives image-heavy carousels a visual navigation option in addition to arrows, dots, and progress indicators, making it easier for users to identify and jump to specific slides.

Type of change

• Bug fix
• New feature
• Enhancement/refactor
• Documentation update
• Test update
• Build/CI/tooling

Related issue(s)

Closes #140

What changed

• Added a new rt-carousel/carousel-thumbnails child block that can be inserted inside carousel blocks.
• Added frontend thumbnail initialization that syncs thumbnail clicks, active state, and thumbnail strip scrolling with the main Embla carousel.
• Added editor thumbnail previews, including image thumbnails when slide images are available.
• Added thumbnail styles for active state, image previews, and non-image fallbacks.
• Added unit tests for thumbnail initialization, click behavior, active state syncing, image rendering, cleanup, and editor behavior.

Breaking changes

Does this introduce a breaking change? If yes, describe the impact and migration path below.

• Yes — migration path:
• No

Testing

Describe how this was tested.

• Unit tests
• Manual testing
• Cross-browser testing (if UI changes)

Test details:

npm run lint:css
npm run lint:js
npm run lint:js:types
npm run test:js -- --runInBand
npm run build

Verified that:

• thumbnails render in the editor and frontend;
• image slides show image thumbnails instead of tiny cloned slide previews;
• clicking a thumbnail changes the active slide;
• active thumbnail state updates when the carousel changes;
• non-image slides still render a fallback preview.

Screenshots / recordings

Add editor and frontend screenshots showing thumbnail navigation.

Checklist

• I have self-reviewed this PR
• I have added/updated tests where needed
• I have updated docs where needed
• I have checked for breaking changes