feat(ui): add compact KPI metrics in tab bar (Total, Err%, Avg, P99)

New TabKpis component shows scope-aware metrics with trend arrows
aligned right in the content tab bar. Each metric shows current value
and an arrow indicating change vs previous period (green=good, red=bad).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/ContentTabs.module.css

@@ -1,5 +1,10 @@
 .wrapper {
-  padding: 0.625rem 1.5rem 0;
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 0 1.5rem;
+  padding-top: 0.375rem;
   border-bottom: 1px solid var(--border);
   background: var(--surface);
+  gap: 1rem;
 }