Add test coverage for the release-checklist demo page's state logic

[Abidoyesimze]

Summary

Beyond the as-any cast tracked separately, the whole page's add/edit/delete-checklist-item and sign-off state logic (442 lines) is entirely untested.

Affected files

• src/app/demo/release-checklist/page.tsx

Tasks

• Add tests for adding, editing, and deleting checklist items.
• Add tests for sign-off status transitions.

Acceptance criteria

• Scope is limited to the files listed (or agreed alternatives in the PR).
• Change is verifiable: tests, screenshots, or written QA steps in the PR.
• No behavior regressions in related user flows.
• No new duplicate abstractions without a one-line rationale in the PR.

Suggested labels

testing

Notes

Sourced from the 2026-07-25 round-2 follow-up audit of the frontend codebase at commit 779e531, run after the first 60-issue audit batch (#525-#584) was substantially resolved. Verified against current file content at time of writing.