fix(e2e): mock /api/invoices/stats/summary to prevent useEffect crash on Invoices page

The GRO-609 paymentStats useEffect fetches /api/invoices/stats/summary
on every render. Without a mock, the response {} (from the generic // Appointments,
clients, ... fallback) doesn't contain revenueThisMonth, causing the page
to fail rendering before AdminLayout ever mounts. Other admin pages don't
have this problem because they don't make unconditional side-effect fetches.

E2E tests mock all /api/** calls, so the new endpoint needs its own mock.

cc @cpfarhood

apps/e2e/tests/navigation.spec.ts

@@ -44,6 +44,16 @@ test.beforeEach(async ({ page }) => {
         json: { newClients: [], activeInPeriodCount: 0, churnRisk: [], churnRiskTotal: 0 },
       });
     }
+    if (url.includes("/api/invoices/stats/summary")) {
+      return route.fulfill({
+        json: {
+          revenueThisMonth: 0,
+          outstanding: 0,
+          refundsThisMonth: 0,
+          methodBreakdown: [],
+        },
+      });
+    }
     if (url.includes("/api/invoices")) {
       return route.fulfill({ json: { data: [], total: 0 } });
     }