Admin UI polish: hub page naming/descriptions, duplicate empty-state message

Problem/Motivation

The AI Metering hub page (/admin/config/ai/ai-metering) has inconsistent naming and outdated/overlong descriptions: "Usage Dashboard" doesn't match "Dashboard by Role"'s naming, that same dashboard's quota mention isn't mirrored on the role dashboard, "drupal/ai Core Settings" is awkwardly worded, and "Personal usage"'s description is a paragraph of placement instructions instead of a short link description.

Proposed resolution

Rename "Usage Dashboard" to "Dashboard by User" (consistent with "Dashboard by Role") and "drupal/ai Core Settings" to "Drupal AI Settings". Shorten and update descriptions:

• Dashboard by User: "Token usage, cost, and quota per user for the current month."
• Dashboard by Role: "Token usage, cost, and quota aggregated by role."
• Drupal AI Settings: "Provider API keys, default models, and drupal/ai configuration."
• Permissions: "Control which roles can view the usage dashboards and administer metering settings." (pluralized: two dashboards now)
• Personal usage: "View your own AI usage and quota for the current month." (drop the block-placement instructions)

On the per-user and by-role dashboards, an empty month shows two near-identical messages stacked: the top-active chart's own empty state ("No AI usage recorded yet for @month.", ai-metering-top-active.html.twig:37) and the main table's ("No AI usage recorded for @month.", ai-metering-dashboard.html.twig:136 / ai-metering-by-role.html.twig:99). Both fire from the same underlying "no usage this month" condition.

Hide the top-active chart section entirely on these two dashboard pages when there are no rows for the selected month, since the main table already shows the empty state. The chart's own empty message stays as-is for standalone block placements, where it's the only indicator.

AI assistance

Yes.