Add searchable user pickers for tasks and report sharing

Replace long scroll-only user lists with searchable user pickers in task assignment and Reports Hub sharing/access permission flows.

Transcript Context

Catherine said teams are struggling with the number of people in user lists for task assignments and report permissions because there is no way to search by name. Users have to scroll through long lists, sometimes across multiple brands or observer accounts, which makes quick assignment and sharing tedious.

Chris agreed this should use an autocomplete-style control instead of a plain select and said both the task assignment flow and the Reports Hub share/access flow are affected. The goal is faster name lookup rather than manual scrolling through dozens of users.