feat(usage): resource usage metering page

Add the Usage page with Summary / Tokens / GPU Instances / Storage / Resource
Events tabs over the new metering endpoints: per-resource breakdowns with
date / scope / user / resource filters, trend charts, server-side sortable
tables (GPU-Hours, Instance-Hours, GB-Days, GB-Hours), Excel export with an
in-dialog preview, and KPI cards with help tooltips explaining each metric.

MaaS-only users (no Kubernetes cluster and no resource events) get a
tokens-only view with the tab bar dropped; GPU Service / the full page unlock
for admins, cluster owners, or anyone who has run a resource. Instance-type
rows reuse the GPU Instances list styling, and deleted users / instances /
volumes are flagged in breakdowns and filters.

src/pages/usage/components/summary-tab.tsx

@@ -423,6 +423,10 @@ const SummaryTab: React.FC = () => {
             headline={
               <>
                 <Stat value={fmt(summary?.gpu_hours)} label="GPU Hours" />
+                <Stat
+                  value={fmt(summary?.instance_hours)}
+                  label="Instance Hours"
+                />
                 <Stat
                   value={computeSum?.active_instances ?? 0}
                   label="Active Instances"