Fix Stripe CDN mock override and free-plan sync guards in E2E tests

- Block real Stripe CDN (js.stripe.com) in injectMockStripe() so the
  addInitScript mock is never overwritten by the async-loaded SDK
- Replace waitForFunction(signUpWithEmail) with waitForLoadState in
  all 8 free-plan auth tests; defer scripts run before DOMContentLoaded
  so the function is guaranteed present without polling for it

tests/e2e/checkout-stripe.spec.ts

@@ -36,6 +36,8 @@ async function injectMockStripe(page: Page, opts: {
   confirmResult?: { error?: { message: string } };
   declineMessage?: string;
 } = {}) {
+  // Block the real Stripe CDN so it cannot override the addInitScript mock
+  await page.route('https://js.stripe.com/**', (route) => route.abort());
   await page.addInitScript((o) => {
     (window as any).Stripe = function (_key: string) {
       const confirmResult = o.declineMessage