feat: add payment page and wire compose submit to payment flow

- api/payment.ts: payOrder(orderId) calls POST /api/payment/{id}/pay
- api/orders.ts: add amountPaid (number|null) to Order type
- PaymentRedirect.vue: route /betalning/:orderId, shows plate from
  query?plate, amount label (49 kr), green Betalt button, mock note:
  "Detta är en mock-betalning. I framtiden skickas du till Stripe."
  On click: calls payOrder, on success navigates to /orders, on
  failure shows error. Button disables and shows "Bearbetar..." while
  paying.
- ComposePage.vue: after createOrder success, captures returned order
  object and navigates to /betalning/{orderId}?plate=... instead of
  the old direct-to-orders route
- Router: add /betalning/:orderId route (name: payment, component:
  PaymentRedirect, meta: { requiresAuth: true })

frontend/src/api/orders.ts

@@ -5,6 +5,7 @@ export interface Order {
   plate: string
   status: string
   trackingId: string | null
+  amountPaid: number | null
   createdAt: string
 }
 

frontend/src/api/payment.ts

@@ -0,0 +1,8 @@
+import { request } from './client'
+import type { Order } from './orders'
+
+export function payOrder(orderId: string): Promise<Order> {
+  return request<Order>(`/payment/${orderId}/pay`, {
+    method: 'POST',
+  })
+}

frontend/src/pages/ComposePage.vue

@@ -34,8 +34,12 @@ async function handleSubmit() {
   errorMessage.value = ''
 
   try {
-    await createOrder(plate.value, letterText.value)
-    await router.push({ name: 'orders' })
+    const order = await createOrder(plate.value, letterText.value)
+    await router.push({
+      name: 'payment',
+      params: { orderId: order.id },
+      query: { plate: plate.value },
+    })
   } catch {
     errorMessage.value = 'Kunde inte skapa beställningen. Försök igen senare.'
   } finally {

frontend/src/pages/PaymentRedirect.vue

@@ -0,0 +1,138 @@
+<script setup lang="ts">
+import { ref } from 'vue'
+import { useRouter, useRoute } from 'vue-router'
+import { payOrder } from '@/api/payment'
+
+const router = useRouter()
+const route = useRoute()
+
+const orderId = route.params.orderId as string
+const paying = ref(false)
+const error = ref('')
+
+async function handlePay() {
+  paying.value = true
+  error.value = ''
+
+  try {
+    await payOrder(orderId)
+    await router.push({ name: 'orders' })
+  } catch {
+    error.value = 'Kunde inte genomföra betalningen. Försök igen.'
+  } finally {
+    paying.value = false
+  }
+}
+</script>
+
+<template>
+  <div class="payment">
+    <h1 class="payment__title">Betalning</h1>
+    <p class="payment__subtitle">
+      Registreringsnummer: <strong>{{ route.query.plate || '—' }}</strong>
+    </p>
+
+    <div class="payment__card">
+      <div class="payment__amount-row">
+        <span class="payment__label">Att betala</span>
+        <span class="payment__amount">49 kr</span>
+      </div>
+
+      <p v-if="error" class="payment__error">{{ error }}</p>
+
+      <button class="payment__button" :disabled="paying" @click="handlePay">
+        {{ paying ? 'Bearbetar...' : 'Betalt' }}
+      </button>
+
+      <p class="payment__note">
+        Detta är en mock-betalning. I framtiden skickas du till Stripe.
+      </p>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.payment {
+  max-width: 28rem;
+  margin: 3rem auto 0;
+  padding: 0 1rem;
+}
+
+.payment__title {
+  margin: 0 0 0.25rem 0;
+  font-size: 1.5rem;
+  color: #1a202c;
+}
+
+.payment__subtitle {
+  margin: 0 0 1.5rem 0;
+  color: #718096;
+  font-size: 0.875rem;
+}
+
+.payment__card {
+  background: #fff;
+  border: 1px solid #e2e8f0;
+  border-radius: 0.75rem;
+  padding: 1.5rem;
+}
+
+.payment__amount-row {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  margin-bottom: 1.25rem;
+  padding-bottom: 1.25rem;
+  border-bottom: 1px solid #e2e8f0;
+}
+
+.payment__label {
+  font-size: 0.875rem;
+  color: #718096;
+}
+
+.payment__amount {
+  font-size: 1.25rem;
+  font-weight: 700;
+  color: #1a202c;
+}
+
+.payment__error {
+  margin: 0 0 0.75rem 0;
+  padding: 0.5rem 0.75rem;
+  background: #fff5f5;
+  border: 1px solid #fed7d7;
+  border-radius: 0.375rem;
+  color: #c53030;
+  font-size: 0.8125rem;
+}
+
+.payment__button {
+  width: 100%;
+  padding: 0.75rem;
+  border: none;
+  border-radius: 0.5rem;
+  background: #48bb78;
+  color: #fff;
+  font-size: 1rem;
+  font-weight: 700;
+  cursor: pointer;
+  transition: background 0.15s;
+}
+
+.payment__button:hover:not(:disabled) {
+  background: #38a169;
+}
+
+.payment__button:disabled {
+  opacity: 0.6;
+  cursor: not-allowed;
+}
+
+.payment__note {
+  margin: 0.75rem 0 0 0;
+  color: #a0aec0;
+  font-size: 0.75rem;
+  text-align: center;
+}
+</style>

frontend/src/router/index.ts

@@ -7,6 +7,7 @@ import RegisterPage from '@/pages/RegisterPage.vue'
 import LoginPage from '@/pages/LoginPage.vue'
 import OrdersPage from '@/pages/OrdersPage.vue'
 import AdminPage from '@/pages/AdminPage.vue'
+import PaymentRedirect from '@/pages/PaymentRedirect.vue'
 import { useAuthStore } from '@/stores/authStore'
 import { getActivePinia } from 'pinia'
 
@@ -36,6 +37,12 @@ const router = createRouter({
       component: AdminPage,
       meta: { requiresAuth: true, requiresAdmin: true },
     },
+    {
+      path: '/betalning/:orderId',
+      name: 'payment',
+      component: PaymentRedirect,
+      meta: { requiresAuth: true },
+    },
     {
       path: '/registrera',
       name: 'register',