Integrate create manual decision with graphql query

What does this MR do and why?

The decision log panel had a "New decision" button and a form, but saving did nothing. The form closed and the decision was lost. The backend mutation to save a decision already existed, but the frontend never called it.

This MR connects the form to the workItemDecisionCreate mutation. A saved decision now appears at the top of the list right away, with no page reload. The form and the card now read the decision text and link from the same fields, so Edit shows the correct data. The card shows one link, labelled "View source". It opens in a new tab for external links, and in the same tab for GitLab links.

P.S Update and delete are still client-only stubs.

References

Related to #628381 (closed)

Screenshots or screen recordings

Recording	Screenshot for toaster

How to set up and validate locally

1. Turn on the decision_log feature flag in the rails console with Feature.enable(:decision_log).
2. Open a work item and open the Decision log panel.
3. Select New decision. Fill in the decision and the context. You can add a link.
4. Save. The decision must appear at the top of the list immediately.
5. Reload the page. The decision must still be there. This shows it was saved on the server.