Add player progress persistence

[nickmeinhold]

Description

Store player progress (completed challenges, points, difficulty preferences) so it persists across sessions.

Acceptance Criteria

• Define data model for player progress
• Add storage backend (Firebase/Firestore recommended)
• Load player progress on connection
• Save progress on challenge completion
• Handle new vs returning players

Data to Persist

• Completed challenge IDs
• Points/score
• Difficulty feedback history
• Hint usage statistics