feat: replace hardcoded courses with real API, search, filters, and pagination

[Xaxxoo]

Summary

• Remove hardcoded course data, fetch from GET /api/v1/courses via TanStack Query useInfiniteQuery
• Add debounced search input with 300ms delay (custom useDebounce hook)
• Add difficulty filter pills (All/Beginner/Intermediate/Advanced)
• Add tag filter chips populated from GET /api/v1/courses/tags (multi-select)
• Add sort selector (Newest/Most Popular/A-Z)
• Add Load More pagination with useInfiniteQuery (appends next page without replacing current list)
• Add loading skeletons via CourseGridSkeleton component
• Add empty state with SearchX icon and Clear filters button when no courses match
• Update CourseCard to show enrollmentCount and isEnrolled badge for authenticated users
• Update CourseCard button to show Continue Learning/Enroll Now/Sign in to enroll

Test plan

• Page loads and shows real courses from the backend
• Search filters results after 300ms debounce
• Difficulty pills filter correctly
• Tag chips filter correctly with multi-select
• Sort selector works
• Load More appends next page without replacing current list
• Empty state appears when no results match filters
• Authenticated users see enrollment badges
• Loading skeletons display while fetching

Closes #221

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