fix: remove redundant @testing-library/dom devDependency

[atomantic]

Summary

• Use fireEvent from @testing-library/react in the three writers-room tests that still imported it from @testing-library/dom.
• Remove the redundant direct @testing-library/dom devDependency from the client manifest and lockfile root entry while retaining its transitive package entries.
• Document the direct dependency removal.

Closes #7015

Validation

• Affected writers-room tests: 3 files, 26 tests passed.
• cd client && npm run lint passed (2536 files).
• cd client && npm run build passed.
• Full cd client && npm test was attempted locally but encountered unrelated existing localhost:3000 connection-refused/time-out failures; the affected tests pass independently.