Implement NotificationBuilder #3095
Description
Summary
Build rich notifications from decrypted messages, including sender name, channel name, and message preview. Handle fallback when data is incomplete.
Acceptance Criteria
- NotificationBuilder class created
- Builds notification from decrypted message
- Truncates long messages appropriately (100 chars?)
- Handles missing metadata gracefully ("Someone", "a channel")
- Builds fallback notification when no message available
- Handles different message types (text, image, file)
- Sets thread identifier for grouping (by channel)
- Unit tests for notification building