Admin messages: show unread counts and improve follow-up filters

[Odzen]

Replaces old issue #1168.

Problem

Admins need a reliable Messages oversight view for mentor/mentee conversations so they can identify users who have not responded and follow up.

The current /messages-details view has conversation rows, partner filtering, message counts, and an unanswered/stale toggle, but it does not clearly expose true unread message counts per user or conversation.

Required behavior

• Show mentor and mentee users/conversations in one admin-accessible view.
• Filter by affiliation partner, including no affiliation and all partners.
• Show true unread message count, not only total messages or stale/unanswered status.
• Make it clear whether unread counts belong to the mentor, mentee, or conversation.
• Keep existing date range, name search, and conversation detail modal behavior working.

Acceptance criteria

• Admin can sort or filter conversations by unread count.
• Admin can search by mentor or mentee name.
• Admin can filter by affiliation partner.
• Conversation detail still opens and shows the full message history.
• Empty and error states are visible on screen and do not disappear automatically.