fix(guest): make Swish QR scannable on guest payment page

The guest payment page (GuestPaymentRedirect.vue) still used the old
non-compliant QR settings that were already fixed in PaymentRedirect.vue
(commit 573153b). Desktop users scanning the QR with the Swish app could
not pay via QR on the guest checkout path.

Root cause: GuestPaymentRedirect.vue was not updated when the auth-path
QR fix was applied. It kept:
  - margin: 2 (half the ISO/IEC 18004 minimum of 4 modules)
  - color #111827 (dark gray; Swish spec requires pure black)
  - width: 224 (borderline for ~80-90 char pre-fill URLs)

Changes:
  - GuestPaymentRedirect.vue: QR options now match PaymentRedirect.vue
    exactly — margin 4, width 288, errorCorrectionLevel 'M',
    color #000000/#ffffff
  - CSS .payment__qr-img: 224px -> 288px to match the larger QR
  - GuestPaymentRedirect.spec.ts: renamed 'renders QR code after loading
    swish info' to 'renders QR code with spec-compliant settings' and
    added assertions verifying margin, width, and color options

Closes #22

frontend/src/__tests__/GuestPaymentRedirect.spec.ts

@@ -124,12 +124,16 @@ describe('GuestPaymentRedirect', () => {
     })
   })
 
-  it('renders QR code after loading swish info', async () => {
+  it('renders QR code with spec-compliant settings', async () => {
     const { wrapper } = await mountPage()
     await vi.waitFor(() => {
       expect(wrapper.find('.payment__qr-img').exists()).toBe(true)
     })
     expect(mockToDataURL).toHaveBeenCalledTimes(1)
+    const options = mockToDataURL.mock.calls[0][1]
+    expect(options.margin).toBe(4)
+    expect(options.width).toBe(288)
+    expect(options.color).toEqual({ dark: '#000000', light: '#ffffff' })
   })
 
   it('renders Swish payment link', async () => {

frontend/src/pages/GuestPaymentRedirect.vue

@@ -52,9 +52,15 @@ onMounted(async () => {
 
     if (swishPaymentUrl.value) {
       qrDataUrl.value = await QRCode.toDataURL(swishPaymentUrl.value, {
-        width: 224,
-        margin: 2,
-        color: { dark: '#111827', light: '#ffffff' },
+        // Swish requires a reliably scannable black-on-white QR. The previous
+        // settings (margin 2, #111827, 224px) produced a 2-module quiet zone
+        // — half the QR spec minimum — which the Swish app's scanner fails to
+        // read when scanning off a screen. Use the spec-compliant 4-module
+        // quiet zone, pure black, and larger modules.
+        width: 288,
+        margin: 4,
+        errorCorrectionLevel: 'M',
+        color: { dark: '#000000', light: '#ffffff' },
       })
     }
   } catch {
@@ -269,8 +275,8 @@ async function confirmPayment() {
 }
 
 .payment__qr-img {
-  width: 224px;
-  height: 224px;
+  width: 288px;
+  height: 288px;
   border-radius: var(--radius-md);
   margin: 0 auto var(--space-sm);
 }