test(payment): add unit tests for buildSwishPaymentUrl and number normalisation

The PR added + stripping to normalizeSwishNumber and the PaymentRedirect
regression assertion verifies QR options, but payment.ts had no dedicated
test file — coverage was only 50% (only the payOrder path exercised via
mocks in PaymentRedirect.spec.ts). This adds a focused spec covering every
normalisation branch (Swedish national, international, + prefix, Swish
Business, whitespace) and URL construction (amount formatting, message
encoding, base URL). Coverage for payment.ts rises from 50% to ~90%.

Why: jocke pointed out that CI was failing on this PR and that I should
always verify CI passes before considering work done. Investigation
showed all frontend steps pass locally (lint, vue-tsc, 277/277 tests,
coverage). The 2h17m CI failure appears to be a transient runner issue
in the backend-coverage step (backend code is unchanged from master,
which passes CI; E2E also passes). This commit re-triggers CI and
fills the + stripping test gap noticed during the investigation.

Changes:
- Add frontend/src/__tests__/payment.spec.ts (8 tests):
  - Number normalisation: Swedish national (07xx), international (4670xx),
    + prefix stripping, Swish Business (123xx), whitespace removal
  - URL construction: amount with two decimal places, message URL-encoding,
    correct Swish C2B base URL
- payment.ts statement coverage: 50% to ~90%
- Total frontend tests: 269 to 277

frontend/src/__tests__/payment.spec.ts

@@ -0,0 +1,52 @@
+import { describe, it, expect } from 'vitest'
+import { buildSwishPaymentUrl } from '@/api/payment'
+
+describe('buildSwishPaymentUrl', () => {
+  it('normalises Swedish national format to international', () => {
+    expect(buildSwishPaymentUrl('0701234567', 49, 'test')).toContain(
+      'sw=46701234567',
+    )
+  })
+
+  it('strips a leading + from international format', () => {
+    const url = buildSwishPaymentUrl('+46701234567', 49, 'test')
+    expect(url).toContain('sw=46701234567')
+    expect(url).not.toContain('sw=%2B')
+    expect(url).not.toContain('sw=+')
+  })
+
+  it('leaves already-international numbers unchanged', () => {
+    expect(buildSwishPaymentUrl('46701234567', 49, 'test')).toContain(
+      'sw=46701234567',
+    )
+  })
+
+  it('leaves Swish Business numbers (123…) unchanged', () => {
+    expect(buildSwishPaymentUrl('1234567890', 49, 'test')).toContain(
+      'sw=1234567890',
+    )
+  })
+
+  it('strips whitespace from the number', () => {
+    expect(buildSwishPaymentUrl('070 123 45 67', 49, 'test')).toContain(
+      'sw=46701234567',
+    )
+  })
+
+  it('includes the amount with two decimal places in amt', () => {
+    expect(buildSwishPaymentUrl('0701234567', 49, 'test')).toContain(
+      'amt=49.00',
+    )
+  })
+
+  it('URL-encodes the message in the msg parameter', () => {
+    const url = buildSwishPaymentUrl('0701234567', 49, 'ABC 123')
+    expect(url).toContain('msg=ABC+123')
+  })
+
+  it('uses the correct Swish C2B base URL', () => {
+    expect(buildSwishPaymentUrl('0701234567', 49, 'test')).toContain(
+      'https://app.swish.nu/1/p/sw/?',
+    )
+  })
+})