feat: implement contextual skeleton loading states for perceived performance (#403)

[shoaib050326]

Summary

Replaces generic ActivityIndicator spinners with content-aware skeleton screens that match each screen's final UI layout, improving perceived performance per issue #403.

Changes

• 5 new skeleton components: CourseViewerSkeleton, QuizSkeleton, SettingsSkeleton, SubscriptionSkeleton, DataGridSkeleton — each mirrors its screen's structural layout using the shared Skeleton primitive
• 4 Suspense fallbacks updated: course-viewer, quiz, settings — now render shapely skeletons instead of spinning circles
• Profile screen: Suspense fallback set to null since MobileProfile self-handles loading via its isLoading prop + built-in skeleton
• Component-level loaders: MobileCourseViewer, SubscriptionManager, AdvancedDataGrid now use skeletons instead of ActivityIndicator+text
• Bug fix: app/quiz.tsx had a duplicate MobileQuizManager declaration and an unimported lazyScreen() call — cleaned up
• Cleanup: Removed unused ActivityIndicator imports and orphaned centerContent/loadingText/loadingContainer styles
• Barrel exports: Added new skeletons + CourseCardSkeleton (was used in screens but not exported) to src/components/mobile/index.ts

Testing

• 8 new tests in tests/components/SkeletonScreens.test.tsx validating all screen skeletons render
• Updated AdvancedDataGrid.test.tsx loading assertion from ActivityIndicator → Animated.View
• All 142 component tests pass; all 502 passing project tests continue to pass

Risk / Rollback

Low risk — purely UI loading states. Rollback: revert to main, the ActivityIndicator paths still exist structurally in LazyScreen.tsx (untouched utility).

Closes #403

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