Retires the device-local `threadListV2Enabled` preference and deletes the
grouped v1 list from Home and the iPad sidebar.

- `HomeScreen` always renders the flat creation-order FlatList; the
  LegendList grouped layout, its group-collapse state, and the
  `collapsedProjectGroups` preference it wrote are gone. The v2 empty
  state now names the environment itself instead of falling through to
  the group-derived v1 empty state.
- `ThreadNavigationSidebar` drops the grouped item arm of
  `SidebarListItem` and its render cases.
- Sort submenus (project/thread) were already hidden whenever the beta
  was on, so they go with it, along with `hasCustomHomeListOptions`,
  `PROJECT_SORT_OPTIONS`, `THREAD_SORT_OPTIONS`, and the thread-sort
  option itself. `projectSortOrder` stays: it still orders the project
  scopes behind the project filter.
- Deletes `homeListItems.ts`, `threadPresentation.ts`,
  `buildHomeThreadGroups`, and the grouped row components
  (`ThreadListGroupHeader`, `ThreadListRow`, `ThreadListShowMoreRow`);
  `PendingTaskListRow` stays, both lists use it.
- Preferences are a plain JSON blob behind a positive allow-list, so
  dropping the two keys needs no migration — they are ignored on load
  and disappear on the next write.

Co-Authored-By: Claude <noreply@anthropic.com>