fix: allow frontend container host in vite preview and update payment E2E tests

fix: add preview.allowedHosts and preview.host to vite.config.ts

Vite preview server blocks requests from non-localhost hosts by default.
In the E2E Docker Compose stack, Playwright accesses the frontend via
http://frontend (container hostname). Without allowedHosts, Vite returns
"Blocked request. This host is not allowed." and the SPA never mounts,
causing all 59 E2E tests to fail with blank pages and missing elements.

- Add preview.host: true (bind to 0.0.0.0)
- Add preview.allowedHosts: ['frontend', 'localhost']

test: update payment-redirect E2E tests to match current UI

The payment page was redesigned to a two-step confirmation flow:
"Jag har betalat" → confirmation → "Ja, jag har betalat". The E2E
tests still referenced the old single-step "Genomför testbetalning"
button and a removed .payment__note CSS class.

- Update 'payment button marks order as paid' to click through both steps
- Rename 'shows mock payment note' to 'shows Swish payment instructions'
  and assert on actual UI elements (Swish label + payment button)

Result: E2E suite now passes 59/59 tests in the Docker Compose CI stack.

frontend/e2e/payment-redirect.spec.ts

@@ -28,7 +28,8 @@ test.describe('Payment redirect', () => {
     await page.getByRole('button', { name: 'Fortsätt till betalning' }).click()
 
     await page.waitForURL(/\/betalning\//)
-    await page.getByRole('button', { name: 'Genomför testbetalning' }).click()
+    await page.getByRole('button', { name: 'Jag har betalat' }).click()
+    await page.getByRole('button', { name: 'Ja, jag har betalat' }).click()
 
     await expect(page).toHaveURL('/orders')
     await expect(page.getByText('DEF456').first()).toBeVisible()
@@ -41,12 +42,13 @@ test.describe('Payment redirect', () => {
     await expect(page).toHaveURL(/\/logga-in/)
   })
 
-  test('shows mock payment note', async ({ page }) => {
+  test('shows Swish payment instructions', async ({ page }) => {
     await page.goto('/compose?plate=GHI789')
     await page.getByLabel('Ditt meddelande').fill('Hej!')
     await page.getByRole('button', { name: 'Fortsätt till betalning' }).click()
 
     await page.waitForURL(/\/betalning\//)
-    await expect(page.locator('.payment__note')).toBeVisible()
+    await expect(page.getByText('Swisha till')).toBeVisible()
+    await expect(page.getByRole('button', { name: 'Jag har betalat' })).toBeVisible()
   })
 })

frontend/vite.config.ts

@@ -18,6 +18,8 @@ export default defineConfig({
   },
   preview: {
     port: 80,
+    host: true,
+    allowedHosts: ['frontend', 'localhost'],
     proxy: {
       '/api': 'http://backend:8080',
     },