Add schema selector to the editor toolbar for schema-aware databases
(PostgreSQL, SQL Server, Oracle). When a schema is selected, the backend
sets the appropriate search_path before executing queries so users no
longer need to write schema-qualified table names.
Refactor all database drivers, query execution, schema operations, and
business logic into a shared `dbx-core` crate. The Tauri desktop app
becomes a thin shell delegating to dbx-core, enabling future web and
CLI frontends to reuse the same core.
t8y2