How do you handle empty states, errors, and edge cases in your designs?

Design them as part of the flow, not as leftovers. For every screen, specify empty, loading, partial, error, and permission-denied states, plus what happens with very long or missing content. Write error messages that say what happened and what to do next in plain language. The polished happy path is the easy part; the states around it are where trust is earned or lost.

I keep a standard list and run every screen against it: empty because you are new, empty because your filter matched nothing, loading, partial data, error, offline, and no permission. Those are seven different screens and they are not interchangeable, though I see them collapsed into one sad illustration all the time. The first-run empty state is where I spend real time, because it is the most-seen screen in the product for anyone deciding whether to stay, and it is a chance to show what good looks like rather than say 'no items yet'. Error copy I write myself and then argue about. It has to say what happened, whether it was their fault or ours, and exactly what to do next. 'Something went wrong' is an admission that we did not bother. And I stress the content: I paste in a sixty-character name and a company with no logo, because real data is always uglier than mock data. Doing this up front saves the engineers guessing and saves me a round of bug tickets that are really design gaps.