Filtering excludedUserIds from <UserSelect/> can create empty pages

### Description
As discovered in #246, if the first page of user results are all excluded IDs, the page will be empty as they are filtered out post-pagination. This is because [Convex filters suck](https://docs.convex.dev/database/reading-data/filters).

This should probably be fixed by abandoning any client-side filtering and instead render the invalid users as disabled or something.