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/LayoutShell.tsx

@@ -265,7 +265,7 @@ function LayoutContent() {
       />
 
       {!isAdminPage && (
-        <ContentTabs active={scope.tab} onChange={setTab} />
+        <ContentTabs active={scope.tab} onChange={setTab} scope={scope} />
       )}
 
       <main style={{ flex: 1, overflow: 'auto', padding: isAdminPage ? '1.5rem' : '0.75rem 1.5rem' }}>