Brings feat/device-sync into main: on Android, each account's address
books and calendars can sync both ways with the phone's Contacts and
Calendar apps (Settings -> Contacts / Calendar -> Sync to this
device), through a "Bulwark" Android account with a contacts and a
calendar sync adapter.
- native side: the account, both sync adapters, provider access scoped
to the account, push routing to the syncs, sync problem notices
- the sync engine in `src/device-sync`, run as a headless task
- the settings sections, sign-out and removal handling, translations
- design, limitations and verification in `docs/device-sync.md`