Memory Leak Fix: ViewPager2 Fragment Management

[PavloNetrebchuk]

Problem
Memory leaks detected across all MainFragment pager screens (LearnFragment, ProgramFragment, NativeDiscoveryFragment, DashboardGalleryFragment, DownloadsFragment, ProfileFragment) due to improper fragment lifecycle management in ViewPager2.

Root Cause
NavigationFragmentAdapter was storing strong references to Fragment instances in ArrayList, preventing garbage collection when fragments were destroyed.

Changes
NavigationFragmentAdapter: Use fragment factories instead of storing instances
MainFragment: Updated to use factory pattern and added adapter cleanup
LearnFragment: Applied same pattern with proper cleanup

Benefits
✅ Eliminates memory leaks across all pager screens
✅ Proper fragment lifecycle management
✅ Lazy fragment creation (only when needed)
✅ Better performance and memory usage

Testing
Memory profiler shows no more leaks in MainFragment pager screens.

BEFORE	AFTER

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[volodymyr-chekyrta]

LGTM 🚀