- Use shared flex-divided-list/items-with-main classes in session
templates so they pick up consistent styling instead of relying on
non-existent flex-list/flex-item-* classes.
- Add CountUserSessionsByUserID helper and use it in admin user view
so we don't materialize the full session list just to compute
total/active counts.
- Bound the VirtualSessionProvider tombstone map by self-expiring
entries via time.AfterFunc, so map size no longer depends on the
session GC interval.
Co-Authored-By: GitHub Copilot (Claude Opus 4.7) <copilot@github.com>