Summary:
Display the xCluster safe time in yb-master xCluster UI page to improve debugging capabilities.

New UI output

{image uri=https://github.com/user-attachments/assets/bcd96f4f-fa5a-44d5-9b50-2d828c43d277, width=500px}

Fixes #28910
Fixes #28806
Jira: DB-18633

Original commit: 533c8203af70452f29120b5379f4f5227e108ee4 / D45287

Test Plan: Manually tested

Reviewers: jhe

Reviewed By: jhe

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D47505