feat(gamification): build subscription gamification system with achievements

[morelucks]

Closes #953

Summary of Changes

1. Gamification Store & Service (, )

• Zustand state management with persistence for user level progression, XP tracking, badge unlocks, streak counts, and reward item claims/redemptions.
• Dynamic XP scaling algorithm and threshold notifications.
• Leaderboard sorting algorithms supporting All-Time, Weekly, and Streak rankings.
• Native social sharing integration for levels, badges, and achievements.

2. Gamification Hub UI (, )

• Fully interactive with level progress bar, tab navigation (Dashboard, Rewards, Leaderboard), claimable reward vouchers, and settings modal.

3. Tests & Documentation (, )

• Unit and component integration tests for and .
• Comprehensive documentation covering architecture, achievement triggers, reward definitions, component layout, and performance benchmarks.

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