Builds on #11903 — Closes #11899

This PR takes the fix from #11903 and improves the JavaScript approach to be less brittle and more maintainable.

Technical

Changes carried over from #11903:

• index.html: Added $:macros.BookPreview('') to render the #bookPreview modal HTML on the homepage so it's available when lazy-loaded carousel buttons are clicked
• BookPreview.html: Added $if ocaid: guard to prevent rendering a broken preview button when no ocaid is provided
• test_home.py: Updated test to account for the new BookPreview macro call on the homepage

Improvements over #11903:

1. Event delegation instead of re-initialization — The original fix imported initPreviewDialogs into lazy-carousel.js and called it after each AJAX load. This creates tight coupling: every future dynamic content loader would need to know about and manually call initPreviewDialogs. Instead, this PR uses a single delegated click handler on document ($(document).on('click.bookPreview', '[data-book-preview]', ...)), so any preview button works automatically regardless of when it's added to the DOM. This eliminates the import and call from lazy-carousel.js entirely.

2. data-book-preview attribute as JS hook — The original fix targeted the CSS class .cta-btn--preview for JS behavior. This PR adds a data-book-preview attribute and targets that instead, separating styling concerns from behavioral hooks. The CSS class remains for styling; the data attribute is the JS contract. This prevents accidental breakage if someone renames or removes the CSS class.

3. Idempotency guard — Uses jQuery event namespacing (.off('click.bookPreview').on('click.bookPreview', ...)) so the handler is safe if initPreviewDialogs() is ever called more than once, preventing duplicate handler stacking.

4. Fixed module loading guard — Changed the dialog module guard selector in index.js from .cta-btn--preview to #bookPreview. The guard runs at page load to decide whether to lazy-import the dialog module. With event delegation, preview buttons don't exist at page load (they come from lazy carousels), but #bookPreview (the modal target) does — so the module loads correctly.

Testing

1. Go to the homepage
2. Scroll to a carousel with a "Preview" button (e.g. Trending, Classic Books)
3. Click the "Preview" button
4. The Internet Archive book reader should open in a modal on the same page
5. Also verify Preview buttons still work on book pages and search results

Stakeholders

@bhardwajparth51 cc: @mekarpeles