Add Spaced Repetition Study System

[pmady]

Description

Implement a spaced repetition system (SRS) for efficient memorization of key concepts.

Requirements

• Algorithm-based review scheduling (SM-2 or similar)
• Flashcard deck management
• Daily review reminders
• Statistics on retention rate
• Import/export decks

Acceptance Criteria

• SRS algorithm implementation
• Card rating system (Again, Hard, Good, Easy)
• Review queue management
• Progress statistics
• Deck sharing capability