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/TabKpis.module.css

@@ -0,0 +1,44 @@
+.kpis {
+  display: flex;
+  align-items: center;
+  gap: 1rem;
+}
+
+.metric {
+  display: flex;
+  align-items: baseline;
+  gap: 0.25rem;
+  white-space: nowrap;
+}
+
+.label {
+  font-size: 0.625rem;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.5px;
+  color: var(--text-muted);
+}
+
+.value {
+  font-family: var(--font-mono);
+  font-size: 0.75rem;
+  font-weight: 600;
+  color: var(--text-primary);
+}
+
+.arrow {
+  font-size: 0.6875rem;
+  font-weight: 700;
+}
+
+.good {
+  color: var(--success);
+}
+
+.bad {
+  color: var(--error);
+}
+
+.flat {
+  color: var(--text-muted);
+}