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/DashboardL1.tsx

@@ -456,14 +456,9 @@ export default function DashboardL1() {
               onItemClick={(id) => navigate(`/dashboard/${id}`)}
             />
           </Card>
-          <div className={styles.punchcardStack}>
-            <Card title="Transactions (7-day pattern)">
-              <PunchcardHeatmap cells={punchcardData ?? []} mode="transactions" />
-            </Card>
-            <Card title="Errors (7-day pattern)">
-              <PunchcardHeatmap cells={punchcardData ?? []} mode="errors" />
-            </Card>
-          </div>
+          <Card title="7-Day Pattern">
+            <PunchcardHeatmap cells={punchcardData ?? []} />
+          </Card>
         </div>
       )}
     </div>