Thread safety and testing of the new Control page

See discussion on #232, in particular this comment: https://github.com/aiidalab/aiidalab-home/pull/232#issuecomment-5117388253

We might also want to improve the threading tests, per https://github.com/aiidalab/aiidalab-home/pull/232#discussion_r3673874593

Start by reading issue #232 and the referenced comment on pull request #232, including the linked threading-test discussion. Inspect the new Control page and the existing threading tests to identify the thread-safety concerns. Done means the relevant concerns are addressed and the threading tests provide stronger coverage.

Vom Indexierungsmodell aus dem Issue-Text verfasst.