After hydrate, scans legacy localStorage keys via buildMigrationPatch
and, if any data is found, pushes one bulk patch to the server,
applies it locally, then removes the legacy keys. Demo accounts skip
the migration entirely. Cancellable on unmount to avoid setState on
unmounted component.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>