Display recent community activity on the homepage.

Acceptance Criteria:

• Add "Recent Activity" section to homepage
• Show mix of: New comments, New reports, New vulnerabilities
• Personalize for logged-in users (prioritize followed entities)
• Show activity cards with type icon, title, actor, timestamp
• Link to source entity
• Limit to last 24 hours or configurable
• Add "View All" link to full activity page