Account
Local-first progress, with one small cross-device sync layer on top.
This keeps signed-out learning unchanged, then adds a minimal magic-link account seam for syncing the same canonical concept-progress snapshot that already drives the progress-aware product surfaces.
AccountOptional sync
Keep signed-out browsing local-first, then add one magic-link account when you want sync.
This browser can keep working with no account. When you request a magic link, the current local concept progress is left in place, then merged into your synced account after the first authenticated sync succeeds.