Moves the credentials module (keyring + AES-256-GCM encrypted file
store for Qobuz credentials and the OAuth token) out of src-tauri into
a new qbz-credentials crate. src-tauri/credentials re-exports it, so the
Tauri app keeps identical behavior while the Slint shell can share the
exact same implementation, encrypted file, and keyring entries.
The module was already fully self-contained (no Tauri, no crate::
references); only the dependency declarations move with it.
cc
