Lazy-load Stripe SDK so it only loads on checkout (#6546)

# Description of Changes

- Stripe SDK (`@stripe/*` + `js.stripe.com/v3`) was loading on every
page; now it only loads when an upgrade/checkout modal actually opens.
- Converted every import site to `React.lazy()` + `<Suspense>`, gated by
the existing `opened` state.
- Adds a Playwright spec that asserts no Stripe requests on landing or
settings.



---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

frontend/editor/src/proprietary/contexts/CheckoutContext.tsx

@@ -4,6 +4,8 @@ import React, {
   useState,
   useCallback,
   useEffect,
+  lazy,
+  Suspense,
   ReactNode,
 } from "react";
 import { useTranslation } from "react-i18next";
@@ -13,7 +15,12 @@ import licenseService, {
   mapLicenseToTier,
   PlanTier,
 } from "@app/services/licenseService";
-import { StripeCheckout } from "@app/components/shared/stripeCheckout";
+
+const StripeCheckout = lazy(() =>
+  import("@app/components/shared/stripeCheckout").then((m) => ({
+    default: m.StripeCheckout,
+  })),
+);
 import { userManagementService } from "@app/services/userManagementService";
 import { alert } from "@app/components/toast";
 import {
@@ -433,16 +440,18 @@ export const CheckoutProvider: React.FC<CheckoutProviderProps> = ({
 
       {/* Global Checkout Modal */}
       {selectedPlanGroup && (
-        <StripeCheckout
-          opened={isOpen}
-          onClose={closeCheckout}
-          planGroup={selectedPlanGroup}
-          minimumSeats={minimumSeats}
-          onSuccess={handlePaymentSuccess}
-          onError={handlePaymentError}
-          onLicenseActivated={handleLicenseActivated}
-          hostedCheckoutSuccess={hostedCheckoutSuccess}
-        />
+        <Suspense fallback={null}>
+          <StripeCheckout
+            opened={isOpen}
+            onClose={closeCheckout}
+            planGroup={selectedPlanGroup}
+            minimumSeats={minimumSeats}
+            onSuccess={handlePaymentSuccess}
+            onError={handlePaymentError}
+            onLicenseActivated={handleLicenseActivated}
+            hostedCheckoutSuccess={hostedCheckoutSuccess}
+          />
+        </Suspense>
       )}
     </CheckoutContext.Provider>
   );