Implement NotificationBuilder

[adrastaea]

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