[HIGH] Schema drops users table and inserts mock profile rows

DROP TABLE IF EXISTS users runs on every first_time_setup(), and three mock profile rows are inserted. UPDATE profile with no WHERE (issue #9) will update all three rows. Mismatch between single-user design and mock data.\n\nFile: Backend/schema.sql, lines 1, 88, 101-104\nFix: Remove mock profile inserts from schema.sql