Optional Sync Demo: Fixed updating schema when moving todos to synced table

[Chriztiaan]

Original issue reported in dart optional sync demo powersync-ja/powersync.dart#174.
This ensures that we correctly copy over the todos with the authed user ID instead of the default local ID.
Instead of copying the local data over to the sync table verbatim (and getting the owner ID from the local table), we use the newly logged in user's ID.

[changeset-bot]

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