For some reason, Table doesn't seem to inherit the environment in some cases. Reapply environment to each TableColumn (only Theme is required). Work around what clearly seems to be a SwiftUI bug. Fixes #872
Finalize basic flow started in #866.
Repurpose LegacyManager as MigrationManager. Present initial migration UI from "+" menu in app home. Different styles: - iOS → Section / ForEach - macOS → Table