Enable code coverage reporting for cypress tests

### Description

We do not currently capture code coverage stats for the cypress tests. This is leaving us with a big gap in our coverage reporting.

Some links for reference: https://docs.cypress.io/guides/tooling/code-coverage

It must follow the approach taken by the [existing coverage tasks](https://github.com/flowforge/flowforge/blob/65703f1fe07bf66a0e3800b7d45bb7f050b292fe/package.json#L49):

1. generate coverage. nyc always calls the file `coverage-final.json` - so needs to be in its own directory, eg `./coverage/reports/frontend-cypress/coverage-final.json`
2. move & rename that file to `./coverage/reports/frontend-cypress-coverage.json`

Then the codecov task should find it and include it in its reporting.

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

Start with package.json and the existing coverage tasks linked in the issue, then review the Cypress code-coverage guide. Trace the Cypress test coverage output and verify that it produces the requested frontend-cypress coverage file and that the existing codecov task includes it.

Written by the indexing model from the issue text.