feat(messages): show message timestamp on hover

[kaizhou-lab]

Summary

• Display the message creation time next to the copy button when hovering over a message
• Shows HH:mm for today's messages, MM-DD HH:mm for older ones
• Uses the same opacity transition as the existing copy button for consistent UX

Closes #1985

Test plan

• Hover over a message and verify the timestamp appears alongside the copy button
• Verify today's messages show HH:mm format
• Verify older messages show MM-DD HH:mm format
• Verify user messages (right-aligned) show time on the correct side
• Verify timestamp hides when cursor leaves the message