Add account management functionality

This is a first pass to implement an account editor loosely in the style of the original software, with support for create/edit/delete operations on user accounts. <img width="1013" alt="Screenshot 2024-06-10 at 3 36 44 PM" src="https://github.com/mierau/hotline/assets/868228/b11dd29f-f77b-4e52-87c5-facf4c5bdbdd"> Any feedback appreciated and no feelings hurt if you want to change anything. ### Rough Edges * No error handling or user feedback if a transaction fails. This is true everywhere, so it'd be good to figure out a general solution to present user facing error. * The presence of a second "Hide Sidebar" button in the toolbar is unwelcome. I may be misusing `NavigationSplitView` by nesting it in a three column layout. * Editing an account and navigating away from it loses the unsaved changes. I'd maybe like to preserve that or prompt for confirmation when clicking away. * The account login field cannot be changed at the moment. This is doable, but requires switching from the simple setUser transaction to the to the more complicated 1.5+ updateUser transaction.