Improve timestamp display in the UI with relative times and formatting.

Acceptance Criteria:

• Display timestamps in "Just now", "5 minutes ago" format
• Show full timestamp on hover
• Add date separators (e.g., "Today", "Yesterday", "May 10")
• Format timestamps based on locale
• Update relative times periodically
• Test across different timezones

Estimated Effort: 1-2 days

Dependencies: None