fix(guest): route homepage CTA to guest checkout when unauthenticated

Guest checkout was unreachable: the homepage CTA "Fortsatt till brevet"
always linked to /compose (requiresAuth), so anonymous users hit the
router guard and got redirected to /logga-in instead of reaching the
guest checkout flow.

Changes:
- HomePage.vue: compute CTA target based on auth state. Authenticated
  users still go to /compose; unauthenticated users go to
  /gast-bestallning with the plate as a query param.
- GuestCheckoutPage.vue: accept plate from route.query.plate so the user
  does not have to re-enter it after looking it up on the homepage.
- HomePage.spec.ts: add Pinia setup + guest-checkout route; update CTA
  assertion to expect /gast-bestallning?plate=... when unauthenticated.

All 318 frontend tests pass, vue-tsc clean (exit 0).

frontend/src/__tests__/HomePage.spec.ts

@@ -1,8 +1,10 @@
-import { describe, it, expect, vi } from 'vitest'
+import { describe, it, expect, vi, beforeEach } from 'vitest'
 import { mount } from '@vue/test-utils'
+import { createPinia, setActivePinia } from 'pinia'
 import { createRouter, createMemoryHistory } from 'vue-router'
 import HomePage from '@/pages/HomePage.vue'
 import ComposePage from '@/pages/ComposePage.vue'
+import GuestCheckoutPage from '@/pages/GuestCheckoutPage.vue'
 
 vi.mock('@/api/vehicles', () => ({
   lookupVehicle: vi.fn(),
@@ -17,6 +19,11 @@ function createTestRouter() {
     routes: [
       { path: '/', name: 'home', component: HomePage },
       { path: '/compose', name: 'compose', component: ComposePage },
+      {
+        path: '/gast-bestallning',
+        name: 'guest-checkout',
+        component: GuestCheckoutPage,
+      },
     ],
   })
 }
@@ -28,6 +35,12 @@ function mountHome(router: ReturnType<typeof createTestRouter>) {
 }
 
 describe('HomePage', () => {
+  beforeEach(() => {
+    setActivePinia(createPinia())
+    localStorage.removeItem('auth_token')
+    vi.clearAllMocks()
+  })
+
   it('renders headline', () => {
     const router = createTestRouter()
     const wrapper = mountHome(router)
@@ -88,7 +101,7 @@ describe('HomePage', () => {
     expect(cta.text()).toBe('Fortsätt till brevet')
   })
 
-  it('CTA links to compose page with plate query param', async () => {
+  it('CTA links to guest checkout with plate query param when unauthenticated', async () => {
     mockLookupVehicle.mockResolvedValue({
       make: 'Volvo',
       model: 'V70',
@@ -107,6 +120,6 @@ describe('HomePage', () => {
 
     const cta = wrapper.find('.btn--primary')
     const href = cta.attributes('href')
-    expect(href).toBe('/compose?plate=ABC123')
+    expect(href).toBe('/gast-bestallning?plate=ABC123')
   })
 })

frontend/src/pages/GuestCheckoutPage.vue

@@ -1,11 +1,12 @@
 <script setup lang="ts">
 import { ref, computed } from 'vue'
-import { useRouter } from 'vue-router'
+import { useRouter, useRoute } from 'vue-router'
 import { createGuestOrder } from '@/api/guestOrders'
 
 const router = useRouter()
+const route = useRoute()
 
-const plate = ref('')
+const plate = ref((route.query.plate as string) || '')
 const letterText = ref('')
 const email = ref('')
 const submitting = ref(false)

frontend/src/pages/HomePage.vue

@@ -1,10 +1,23 @@
 <script setup lang="ts">
-import { ref } from 'vue'
+import { ref, computed } from 'vue'
 import { RouterLink } from 'vue-router'
 import PlateInput from '@/components/PlateInput.vue'
 import VehicleInfo from '@/components/VehicleInfo.vue'
 import type { VehicleInfo as VehicleData } from '@/components/VehicleInfo.vue'
 import { lookupVehicle } from '@/api/vehicles'
+import { useAuthStore } from '@/stores/authStore'
+
+const auth = useAuthStore()
+
+// When unauthenticated, the homepage CTA sends the user to the guest
+// checkout page instead of the auth-protected /compose route.
+// Without this, anonymous users hit the router guard and are bounced
+// to /logga-in — the guest flow we built was unreachable from the UI.
+const ctaTarget = computed(() =>
+  auth.isAuthenticated
+    ? { name: 'compose' as const, query: { plate: plate.value } }
+    : { name: 'guest-checkout' as const, query: { plate: plate.value } },
+)
 
 const useCases = [
   {
@@ -204,7 +217,7 @@ async function handleLookup(lookedUpPlate: string) {
 
         <RouterLink
           v-if="vehicle"
-          :to="{ name: 'compose', query: { plate } }"
+          :to="ctaTarget"
           class="btn btn--primary btn--lg home__cta"
         >
           Fortsätt till brevet