Replace all JSON file storage (connections, secrets, history, AI config,
conversations, sidebar layout) with a single SQLite database (dbx.db).
Auto-migrates existing JSON data on first launch, renaming old files to
.bak. Remove keyring dependency — passwords now stored in SQLite.
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