Add cleanups to `useEffect()` calls with callbacks

There are a few useEffect() calls that take a promise and call then() on it. (These are in TaskTitle(), AccountName(), and usePromise()). There's also one in ErrorPage, but it's moot because the effect navigates away from the app.) These should return cleanup functions, following the usual cancellation flag pattern. It's possible that rapid clicks, plus HTTP requests completing out of order, could lead to confusing UI interactions, and ignoring promise resolutions from old invocations of effects will fix these potential issues.

Locate the useEffect calls in TaskTitle(), AccountName(), and usePromise(), and inspect how their promises call then(). Follow the usual cancellation-flag cleanup pattern so resolutions from older effect invocations are ignored; ErrorPage is out of scope because its effect navigates away. Done means each listed effect returns cleanup behavior that prevents stale rapid-click results from affecting the UI.