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/portal-data.spec.ts

@@ -72,9 +72,15 @@ test.describe("Portal Data Integrity", () => {
   });
 
   test("billing section renders without JS errors", async ({ page }) => {
-    // Mock billing endpoint
-    await page.route("**/api/billing**", (route) =>
-      route.fulfill({ json: { invoices: [], balanceCents: 0 } })
+    // Mock portal billing endpoints
+    await page.route("**/api/portal/config**", (route) =>
+      route.fulfill({ json: { stripePublishableKey: "" } })
+    );
+    await page.route("**/api/portal/invoices**", (route) =>
+      route.fulfill({ json: [] })
+    );
+    await page.route("**/api/portal/payment-methods**", (route) =>
+      route.fulfill({ json: [] })
     );
 
     const consoleErrors: string[] = [];