Split Monolithic Auth Store

[baberlabs]

Problem:

The useAuthStore handles authentication, socket management, online users, and error handling all in one store, violating single responsibility principle.

Why it's an issue:

• Hard to test individual features
• Tight coupling between unrelated concerns
• State updates can cause unnecessary re-renders

Acceptance Criteria:

• Split auth store into separate domain-specific stores
• Create dedicated socket store
• Maintain same functionality with better separation