Prebuilt table preview performance improvements

[erdembircan]

Background: Loading pre-built table previews on frontend hangs editor and takes a significant time.

Solution Summary:

• Added a loading indicator to individual prebuilt cards for server fetch operations.
• Added lazy load system for previews of those templates where now only template cards in view will be rendered. This way whole list is lighting fast and more responsive.

Known Limitations
The core of this preview system relies on WordPress's native component, which prevents ref-based approaches from functioning as intended. Specifically, the preview component loads a dummy webpage into an iframe, but without a callback for the page load completion event, the exact timing of the rendering operation cannot be pinpointed. Until WordPress provides such a callback, using a lazy loading system based on visibility remains the most accurate workaround for this issue.

Demo