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-lock.json

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         "@react-pdf/renderer": "^4.5.1",
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         "react": "19.2.4",
-        "react-dom": "19.2.4"
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+        "stripe": "^22.3.2"
       },
       "devDependencies": {
         "@tailwindcss/postcss": "^4",
@@ -3330,6 +3333,29 @@
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       "license": "MIT"
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+      "license": "MIT",
+      "dependencies": {
+        "prop-types": "^15.7.2"
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+      "peerDependencies": {
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+        "react": ">=16.8.0 <20.0.0",
+        "react-dom": ">=16.8.0 <20.0.0"
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+    },
+    "node_modules/@stripe/stripe-js": {
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+      "license": "MIT",
+      "engines": {
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@@ -11285,6 +11311,23 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
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+    "node_modules/stripe": {
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     "node_modules/strtok3": {
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