feat: consolidate punchcard heatmaps into single toggle component

Replace two separate Transaction/Error punchcard cards with a single
card containing a Transactions/Errors toggle. Uses internal state to
switch between modes without remounting the chart.

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

ui/src/pages/DashboardTab/DashboardTab.module.css

@@ -140,6 +140,37 @@
   gap: 16px;
 }
 
+/* Toggle button row */
+.toggleRow {
+  display: flex;
+  gap: 2px;
+  padding: 0 12px 4px;
+}
+
+.toggleBtn {
+  padding: 3px 10px;
+  font-size: 11px;
+  font-family: var(--font-mono);
+  color: var(--text-muted);
+  background: transparent;
+  border: 1px solid var(--border-subtle);
+  border-radius: var(--radius-sm);
+  cursor: pointer;
+  transition: all 0.15s;
+}
+
+.toggleBtn:hover {
+  color: var(--text-primary);
+  border-color: var(--border);
+}
+
+.toggleActive {
+  color: var(--text-primary);
+  background: var(--bg-inset);
+  border-color: var(--border);
+  font-weight: 600;
+}
+
 /* Errors section */
 .errorsSection {
   background: var(--bg-surface);