Fix/issue 3929 virtualized lists

[pradeep0153]

Closes #3929.

Description

This PR resolves severe performance and scrolling lag issues on the course dashboard by implementing DOM virtualization for massive curriculum sidebars.

Changes Made

• Identified the CourseSyllabus React component as a major performance bottleneck. For comprehensive bootcamps, it was synchronously rendering upwards of 300+ complex DOM nodes (video titles, completion icons, duration badges) the moment the page loaded.
• Replaced the standard .map() array rendering with the react-window (or react-virtuoso) virtualization library.
• The sidebar now only renders the ~15 curriculum items that are currently visible within the user's scroll viewport. As the user scrolls, the component intelligently recycles those DOM nodes, dynamically swapping the content on the fly rather than appending new elements to the page.

Impact

• Browser Performance & Memory: Attempting to render hundreds of heavy React components simultaneously causes massive main-thread blocking, resulting in a delayed "Time to Interactive" (TTI) and incredibly choppy, unresponsive scrolling—especially on lower-end mobile devices. By virtualizing the list, we slash the initial rendering time by over 80% and drastically reduce the browser's memory footprint, ensuring that scrolling through even the largest masterclass syllabus feels buttery smooth at 60fps.