[Feature] Implement Email and In-App Notification Preferences Per User

[knoxiboy]

Description
Users likely receive all notifications or none. Add granular notification preferences that let each user choose which events trigger notifications (task assigned, comment mentioned, sprint deadline) and via which channel (in-app, email).

Motivation
Notification overload causes users to ignore all notifications. Granular preferences ensure users see what matters to them and nothing else.

Implementation

1. Add a notification_preferences table in Supabase: user_id, event_type, channel, enabled.
2. Build a Notification Settings page with toggle switches for each event/channel combination.
3. Update the notification dispatch logic to check preferences before sending.
4. Default all preferences to "enabled" for new users.

Level: Advanced | Affected Files: src/, supabase/

[knoxiboy]

/assign gssoc'26
@Priyanshu-byte-coder Please assigne this issue to me.