Previously, cancelling a DuckDB query only dropped the tokio CancellationToken,
but the actual DuckDB C call continued running in spawn_blocking. This caused
the UI to show "正在停止..." until timeout silently expired.
Now, RunningQueries stores per-execution interrupt callbacks. For DuckDB and
ExternalTabular branches, an InterruptHandle is registered on query start so
cancel() calls duckdb_interrupt() at the C level, truly stopping the query.
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.