feat(ui): sort session history by last activity instead of creation time

[Raghavendiran-2002]

Summary

Closes #1556

Sessions in the history sidebar were sorted by creation time, making it hard to find active long-running sessions.
This change sorts by last activity (updated_at) instead.

• Grouping (Today / Yesterday / Older) now uses updated_at so a session active today appears under "Today" even
  if it was created days ago
• Sort order within each group is descending by updated_at — most recently active session appears first
• Timestamp displayed on each session item now shows last activity time instead of creation time
• Falls back to created_at if updated_at is not set

Files changed

• ui/src/components/sidebars/GroupedChats.tsx — grouping and sort logic
• ui/src/components/sidebars/SessionGroup.tsx — pass updatedAt to ChatItem
• ui/src/components/sidebars/ChatItem.tsx — display updatedAt timestamp

[Copilot]

Pull request overview

This PR improves the session history sidebar by sorting sessions by their last activity time (updated_at) instead of creation time. This makes it easier to find active long-running sessions. The implementation includes proper grouping (Today/Yesterday/Older) and sorting (most recently active first) using the updated timestamp, with a fallback to created_at for backward compatibility.

Changes:

• Updated session grouping logic to use updated_at instead of created_at
• Updated session sorting within each group to use updated_at in descending order
• Modified ChatItem component to accept and display updatedAt timestamp instead of createdAt

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
ui/src/components/sidebars/GroupedChats.tsx	Updated grouping and sorting logic to use updated_at with fallback to created_at
ui/src/components/sidebars/SessionGroup.tsx	Updated prop passed to ChatItem from createdAt to updatedAt
ui/src/components/sidebars/ChatItem.tsx	Updated interface and display to use updatedAt instead of createdAt

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[Copilot]

Note: The ChatItem component's prop has been changed from createdAt to updatedAt, but the Storybook stories file (ChatItem.stories.tsx) still references the old createdAt prop in 4 places (lines 65, 76, 87, and 110). This file will need to be updated separately to reflect the prop name change.