Add QR scanner E2E tests

### What is this about?

Add E2E tests for the QR scanner feature. This is challenging right now because we haven't yet established a way of mocking webcam input during e2e tests (though it appears to be possible with Chrome+Selenium at least, unsure about Firefox).

### Scenario

_No response_

### Design

_No response_

### Technical Details

- At least one E2E test has been introduced that shows the QR scanner works correctly
- At least one E2E test has been written that ensures #28527 has been fixed

### Threat Modeling Framework

_No response_

### Acceptance Criteria

_No response_

### Stakeholder review needed before the work gets merged

- [X] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [X] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

_No response_

Start by reviewing the QR scanner implementation and issue #28527, then inspect the repository's existing E2E setup. Investigate how webcam input can be mocked in Chrome and Firefox; done means adding one test that verifies the scanner and another confirming the fix for #28527.

Written by the indexing model from the issue text.