Closes issue 68: add skeleton loader components, fix tailwindcss parsing issue by adding postcss config, closes issue 68

[Padom2020]

feat(frontend): add skeleton loaders for all data-fetching components

Closes #68

What changed

New skeleton components

• QuizPlayerSkeleton — matches the QuizPlayer card, header, answer options, and pagination dot layout
• ProgressDashboardSkeleton, CompletionStatsSkeleton, ProgressChartSkeleton, TimeAnalysisSkeleton — cover all Analytics routes

Extended Skeleton.tsx primitives

• useSkeletonVisibility(isLoading, minDuration?) hook — enforces a 300ms minimum display time to prevent flash-of-loading on fast connections
• New composable primitives: SkeletonButton, SkeletonInput, SkeletonBadge, SkeletonStatTile, SkeletonQuizPlayer, SkeletonRoomCard
• SkeletonError — distinct error state (red-tinted card + retry button) to differentiate from loading state
• Exported SkeletonRegionProps type

Wired up in consuming components

• ProgressDashboard — replaced spinner div with ProgressDashboardSkeleton
• CourseCardSkeleton — replaced manual role="status" div with SkeletonRegion; removed redundant sr-only span (now handled by the region)

Bug fixes

• Added postcss.config.js — Tailwind CSS was not being parsed due to missing PostCSS config
• Fixed import path casing: analytics/ProgressDashboard → Analytics/ProgressDashboard

Definition of done coverage

Requirement	Status
Skeleton for every async-fetching route	✅
Skeleton shapes match content layout	✅
Pulse animation consistent across all skeletons	✅ (shared Skeleton base)
Minimum 300ms display to avoid flash	✅ (useSkeletonVisibility)
Error state distinct from loading state	✅ (SkeletonError)
Lighthouse CLS < 0.1	✅