Enum/boolean cell dropdowns use the native OS <select> (unstyled, broken on Linux) #28

Editing an enum cell (and the enum/boolean fields in the new-row draft) uses a native HTML <select>. Its popup is rendered by the OS, so it ignores the app's dark theme (white box) and renders/behaves poorly on Linux/WebKitGTK.

Expected
A themed, portaled dropdown that looks and behaves the same on macOS, Windows, and Linux.

Addressed by #21.