Add social features

[mondominator]

Description

Add social elements to enhance user engagement.

Features

• Written reviews (not just ratings)
• Share current book/progress
• Public reading lists
• Follow other users
• Activity feed
• Book discussions

Implementation

• Review UI in book details
• Share sheets for progress
• Public profile option
• Social API endpoints

Acceptance Criteria

• Can write text reviews
• Can share progress to social media
• Can make profile public
• Can follow other users
• Can see activity feed
• Privacy controls

Priority: 🟢 Low