feat(math-flashcards): implement avatar, sharing, streaks, and data import

[kkd16]

This PR introduces several new features to the Math Flashcards app to improve user engagement and data portability.

1. Social Sharing: A "Share to X" button has been added to the game summary screen, allowing users to quickly tweet their score and difficulty.
2. Custom Avatars: Users can now set a custom emoji/text avatar in the settings menu. This avatar is displayed prominently in the header and persists across sessions via localStorage.
3. Daily Streak: A daily streak counter has been added. It tracks consecutive days played using UTC calendar dates to avoid time-zone edge cases. The current streak is displayed in the main stats view.
4. Data Import: To complement the existing export functionality, an "Import JSON" button is now available in the history section. Users can select a previously exported .json file to restore their history, favorites, run scores, and lifetime stats.
5. JOURNAL.md: 12 new ideas were added to the backlog, and the 4 implemented features were marked as complete.

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PR created automatically by Jules for task 17332084061001559398 started by @kkd16

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[github-actions]

✅ Scope + lint + build passed.