fix(E2E): add missing API mocks for invoices stats and portal billing

navigation.spec.ts:
- Add mock for /api/invoices/stats/summary returning the shape
  { revenueThisMonth, outstanding, refundsThisMonth, methodBreakdown }
  that InvoicesPage useEffect fetches on mount

portal-data.spec.ts billing test:
- Replace incorrect /api/billing** mock with correct portal endpoint
  mocks: /api/portal/config, /api/portal/invoices, /api/portal/payment-methods
  These are the actual endpoints BillingPayments component calls

Both fixes address the E2E failures reported by Lint Roller on PR #348.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/e2e/tests/navigation.spec.ts

@@ -47,6 +47,11 @@ test.beforeEach(async ({ page }) => {
     if (url.includes("/api/invoices")) {
       return route.fulfill({ json: { data: [], total: 0 } });
     }
+    if (url.includes("/api/invoices/stats/summary")) {
+      return route.fulfill({
+        json: { revenueThisMonth: 0, outstanding: 0, refundsThisMonth: 0, methodBreakdown: [] },
+      });
+    }
     // Appointments, clients, services, staff, book, etc.
     return route.fulfill({ json: [] });
   });