Fix: Preview button on homepage does nothing when clicked

[bhardwajparth51]

Closes #11899

Technical

This fix implements the approach suggested by @mekarpeles in #11899,
with some additional improvements.

Root causes:

1. The #bookPreview modal HTML was never rendered on the homepage
   since lazy carousels do not call the BookPreview macro server-side.
2. initPreviewDialogs binds click handlers at page load, but carousel
   content is injected via AJAX afterward, so buttons never received handlers.

Changes:

• index.html: Added $:macros.BookPreview('') to render modal HTML on
  page load so it is available when carousel buttons are clicked
• dialog.js: Added context = document parameter to initPreviewDialogs
  and scoped selector to $(context).find('.cta-btn--preview').not('.cboxElement')
  to prevent double-binding on already initialized buttons
• lazy-carousel.js: Import and call initPreviewDialogs(newElem) after
  AJAX carousel content is inserted into the DOM

Testing

1. Go to https://openlibrary.org/
2. Find a book on the homepage carousel with a "Preview" button
3. Click the "Preview" button
4. The Internet Archive book reader should open inline on the same page

Stakeholders

@mekarpeles @jimchamp @RayBB

[bhardwajparth51]

Hi @mekarpeles

Just a gentle follow-up on this PR since #11899 is marked Priority 1.
It implements the approach you suggested and keeps the changes minimal and scoped to the homepage preview behaviour.
It also looks like two workflows are awaiting maintainer approval -- would appreciate a quick look when you have bandwidth.
Happy to make any adjustments if needed. Thanks

[bhardwajparth51]

@RayBB @jimchamp all test cases are passing now. Would appreciate a review when you have bandwidth. Thanks