Retires the `sidebarV2Enabled` beta flag and deletes the v1 sidebar.

- `AppSidebarLayout` always renders `SidebarV2`; the settings-route nav
  moves into `SidebarV2` (it lived in v1 only, which is why v1 stayed
  mounted on `/settings*`).
- `data-sidebar-version` is gone: both palettes were already identical,
  so the sidebar tokens key off `data-app-sidebar` instead.
- `chat.new` routes through the command palette whenever there is more
  than one project group, which was the v2 branch.
- Drops the flag from `ClientSettingsSchema`/`ClientSettingsPatch`.
  Stored settings that still carry it decode fine (excess keys are
  ignored), covered by a regression test.
- Removes `sidebarThreadPreviewCount`, whose only reader and only UI
  were in the v1 sidebar, along with the now-orphaned
  `preview/openDiscoveredPort.ts`.

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