Replace books and submit with a single sync use-case: pending invoices are
pushed and AADE is mirrored back by MARK cursor in both directions, with
expenses as a first-class entity, MCP Apps widgets, and a repair mode.

Breaking: existing databases are incompatible, YOURDATA_MCP_ALLOW_SUBMIT is
now YOURDATA_MCP_ALLOW_SYNC, and the submit_invoice tool is now sync_now and
push_invoice.