fix: improve Herman portrait framing on the team page

[chengjiahao1234]

Type of Change

• Bug Fix

Summary

Fix Herman Tcheneghon Motcheyo's portrait framing on the team page so his full head and shoulders are visible.

Changes Made

• _data/phd_students.yml: set Herman's portrait position to center top and document the optional field.
• _includes/team-grid.html: support an optional per-member photo_position, while preserving centered cropping for other members and fallback images.
• Keep the original portrait, team ordering, contact details, and profile links unchanged.

Testing Checklist

• Desktop (1920x1080)
• Tablet (768x1024)
• Mobile (375x667)
• Light theme
• Dark theme
• All team images load; existing link targets remain unchanged
• No browser console errors or layout overflow
• Ran bundle exec jekyll serve successfully
• Passed YAML regression checks and git diff --check
• Verified other members retain their existing portrait positioning

Screenshots

Before/after and responsive screenshots were captured locally; they are not attached to this PR.

Related Issues

None.

Additional Notes

This changes the displayed crop only; it does not modify the source photograph. The optional positioning field is documented in the YAML and shared include.

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By submitting this PR, I confirm:

• I have tested these changes locally
• I have followed the code standards in DEVELOPMENT.md
• I have updated documentation where needed
• I am ready for this to be reviewed and merged