ref PROD-708

- added user avatars with contextual icons on singular notifications to
help users quickly identify both the person and type of interaction
- implemented larger icons for grouped notifications to improve
scanability when multiple notifications are bundled together
- emphasized notification content (such as reply text) to focus user
attention on the most relevant information rather than secondary elements
- created visual distinction between posts and notes to help users
understand different content types
- added timestamps to show when each notification occurred