Add Stripe payment processing (cards + PayPal) with a webhook-gated checkout flow

Checkout now branches on payment-methods.provider: Überweisung stays
immediate/unchanged, Kreditkarte/PayPal creates a pending_payment order,
mounts Stripe's Payment Element, and defers invoice/email to a webhook-
verified confirm-payment call once the backend actually confirms payment.
Includes a PAYMENT_TEST_MODE mock provider so the whole gated pipeline is
exercisable locally without a real Stripe account.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -14,11 +14,14 @@
     "@payloadcms/live-preview-react": "^3.85.2",
     "@payloadcms/richtext-lexical": "^3.85.2",
     "@react-pdf/renderer": "^4.5.1",
+    "@stripe/react-stripe-js": "^6.8.0",
+    "@stripe/stripe-js": "^9.12.0",
     "motion": "^12.42.2",
     "next": "16.2.9",
     "nodemailer": "^9.0.3",
     "react": "19.2.4",
-    "react-dom": "19.2.4"
+    "react-dom": "19.2.4",
+    "stripe": "^22.3.2"
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4",