Backend and frontend hardening and polish

[sayed200350]

Implements backend hardening, server intelligence, and data lifecycle features to improve security, enable server-side processing, and enhance user experience.

This PR centralizes critical backend logic in Cloud Functions for enhanced security, including input sanitization, rate-limiting, and abuse protection for community interactions. It also introduces server-side intelligence for pattern detection and automated daily challenge generation. Client-side updates integrate with these new functions, provide deep-link navigation from notifications, and add user data management capabilities.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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