GRO-607: Replace mock payment flow with real Stripe Elements

- Install @stripe/stripe-js and @stripe/react-stripe-js
- Replace BillingPayments mock delay with real Stripe Elements:
  - Fetch publishableKey from GET /api/portal/config
  - Lazy load Stripe via loadStripe()
  - Wrap payment modal in <Elements> with PaymentElement
  - Use stripe.confirmPayment() with clientSecret from pay/pay-multiple endpoints
  - Support multi-invoice selection and single invoice payment
- Add "Save card for future payments" checkbox (setup_future_usage)
- Add payment method management: list saved cards, delete via DELETE endpoint
- Proper error handling for payment failures
- Autopay toggle (UI-only, Phase 2 backend pending)

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

pnpm-lock.yaml

@@ -40,6 +40,9 @@ importers:
       nodemailer:
         specifier: ^6.9.16
         version: 6.10.1
+      stripe:
+        specifier: ^22.0.0
+        version: 22.0.1(@types/node@22.19.15)
       telnyx:
         specifier: ^6.41.0
         version: 6.41.0(ws@8.19.0)
@@ -4167,6 +4170,15 @@ packages:
   strip-literal@3.1.0:
     resolution: {integrity: sha512-8r3mkIM/2+PpjHoOtiAW8Rg3jJLHaV7xPwG+YRGrv6FP0wwk/toTpATxWYOW0BKdWwl82VT2tFYi5DlROa0Mxg==}
 
+  stripe@22.0.1:
+    resolution: {integrity: sha512-Yw764pZ6s8Xu4CtUZdD5uWOkw6gc9xzO9OKylCuj1gMhMDLbyGbDtaPNNSFE4mB6njYSHESYIVbF1iIzUfAl2g==}
+    engines: {node: '>=18'}
+    peerDependencies:
+      '@types/node': '>=18'
+    peerDependenciesMeta:
+      '@types/node':
+        optional: true
+
   strnum@2.2.1:
     resolution: {integrity: sha512-BwRvNd5/QoAtyW1na1y1LsJGQNvRlkde6Q/ipqqEaivoMdV+B1OMOTVdwR+N/cwVUcIt9PYyHmV8HyexCZSupg==}
 
@@ -8857,6 +8869,10 @@ snapshots:
     dependencies:
       js-tokens: 9.0.1
 
+  stripe@22.0.1(@types/node@22.19.15):
+    optionalDependencies:
+      '@types/node': 22.19.15
+
   strnum@2.2.1: {}
 
   supports-color@7.2.0: