Issue #3613292: Hub page naming/descriptions cleanup, duplicate empty-state message
Renamed "Usage Dashboard" to "Dashboard by User" and "drupal/ai Core Settings" to "Drupal AI Settings" on the AI Metering hub page for naming consistency; shortened and updated several descriptions (added quota to the by-role dashboard, pluralized "dashboards" on Permissions, replaced the Personal usage paragraph with a short link description).
Also fixed a duplicate empty-state message: on an empty month, both the per-user and by-role dashboards showed the top-active chart's own "no usage" message stacked on top of the table's identical message. The chart section is now hidden on both dashboard pages when there are no rows for the selected month.
Closes #3613292