Achievements and streaks based on play time

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The issue body is empty, so start by examining the repository structure to understand how play time is tracked. Look for existing data models in Rust backend code (likely in src/) and any frontend components in React that display user stats. Check for existing achievements or streak logic. 'Done' means implementing a system to calculate and display achievements/streaks based on accumulated play time across game sessions.

Written by the indexing model from the issue text.