Fix: More explicit message for duplicate timepoint

[arnav-makkar]

Brief summary of changes

Updated the error message displayed when attempting to create a duplicate timepoint to be more explicit and user-friendly.

Before:

This visit label is not unique.

After:

A timepoint with these conditions (PSCID, Project, Cohort, Visit) already exists.

The new message now includes:

• Participant PSCID
• Project name
• Cohort title
• Visit label

Changes made:

• Modified TimePoint::isValidVisitLabel() to construct a detailed error message
• Updated translation files (.pot, .po) for English, Spanish, French, and Japanese

Link(s) to related issue(s)

• Resolves [create_timepoint] more explicit messages #9776