feat: infinite scroll on course listing pages

[ifygreg01-best]

Summary

Replace traditional pagination with IntersectionObserver-based infinite scroll on the course listing page for a smoother browsing experience.

Changes

New hooks

• useInfiniteScroll — attaches an IntersectionObserver to a sentinel element at the bottom of the list; fires onLoadMore when it enters the viewport (with 300 px pre-load margin). Guards against duplicate calls while a fetch is in flight.
• useVirtualList — windowed rendering for large datasets; only renders the visible slice ± overscan buffer, reducing DOM nodes significantly.
• useScrollRestoration — persists scroll position to sessionStorage, keyed by Next.js pathname or a custom key, and restores it on remount (e.g. navigating back to the listing).

New component

• InfiniteScrollList — composes the three hooks above. Includes:
  • Loading spinner at the bottom during page fetches
  • Skeleton grid on initial load (no layout shift)
  • Empty state with optional clear-filters action
  • Error state with retry button (both full-page and inline for pagination failures)
  • End-of-list indicator once all results are loaded
  • Optional virtualized mode for datasets ≥ 40 items

Updated files

• CourseGrid.tsx — removes the old "Load more" button; now drives InfiniteScrollList with inline search, sort, and category controls.
• discoveryApi.ts — adds listCourses() with CourseListParams / CourseListResponse types supporting cursor-based pagination.
• backend/src/routes/courses.js — adds GET /api/courses listing endpoint supporting both cursor (?cursor=<token>) and offset (?page=2) pagination. Validated via Joi + validateRequestSchema. Returns { items, total, page, limit, hasMore, nextCursor }.

What was tested

• TypeScript: npx tsc --noEmit --skipLibCheck passes with zero errors on all new/modified files.
• Backend syntax: node --check backend/src/routes/courses.js passes.
• Manual checklist:
  • Scroll to bottom triggers next page automatically
  • Filter/sort changes reset to page 1 and refetch
  • Back-navigation restores scroll position
  • Error state shows retry button
  • Empty state shows clear-filters action
  • Skeleton grid shown on initial load (no layout shift)

Labels

area: frontend performance enhancement priority: medium

closes #196