Phase 2: Notification preferences and in-app notifications

[tessak22]

What

Add a notification system so team members are alerted about events that need their attention without having to check the dashboard.

Scope

• Notification preferences in Settings: toggle email/in-app for each event type
• Event types: close-the-loop reminders due, context update proposals pending, session shared with you, artifact status changed
• In-app notification bell in the header with unread count
• Notification dropdown showing recent events with links to relevant pages
• Email notifications via Supabase or a transactional email service (Resend, Postmark)
• Mark as read / dismiss

Reference

SPEC.md Section 4.7 (P2)