Admin account reports: finish message-count, affiliation, sorting, and export parity

[Odzen]

Replaces old issue #1167.

Problem

Admin account reports need consistent operational columns in both the table and Excel exports. Some of this exists today, but the requested behavior should be verified and completed end to end.

Required behavior

For mentor and mentee account reports:

• Show total messages sent.
• Show total messages received.
• Show affiliation partner name when the user is assigned to a partner.
• Include the same fields in Excel downloads.
• Support sorting on the new columns where the UI table supports sorting.

Acceptance criteria

• Mentor and mentee account tables show message sent/received counts and affiliation.
• Excel exports include the same values with clear column names.
• Sorting works for message count columns in the UI.
• Filtered account exports still match the visible filtered rows.