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>

apps/web/package.json

@@ -14,6 +14,8 @@
   },
   "dependencies": {
     "@groombook/types": "workspace:*",
+    "@stripe/react-stripe-js": "^6.1.0",
+    "@stripe/stripe-js": "^9.1.0",
     "@tailwindcss/vite": "^4.2.2",
     "better-auth": "^1.5.6",
     "lucide-react": "^0.577.0",