Add Storage API persistence to protect calendar data

[prybalko]

Add Storage API persistence to protect calendar data

Summary

Requests persistent storage using navigator.storage.persist() to reduce the risk of browsers automatically clearing user calendar data.

Changes

• Added useEffect hook in CalendarContext.tsx to request persistent storage on app load
• Logs persistence status to console for debugging
• Gracefully handles browsers that don't support the Storage API

Benefits

• Browser is less likely to evict localStorage data under storage pressure

Notes

Browsers typically grant persistence based on user engagement (bookmarking, frequent visits, PWA installation). The app continues to work normally whether persistence is granted or denied.