chore(deps): migrate keyring from 3 to 4

keyring v4 is no longer a library crate; the API moved to keyring-core
plus per-platform credential store crates. Replace the keyring dependency
with keyring-core and the native store crates, and register the default
store at runtime (lazily, once) since v4 no longer selects it at compile
time via Cargo features.

src/system/keys.rs

@@ -6,7 +6,7 @@
 pub mod filestorage;
 pub mod keyringstorage;
 // ext
-use keyring::Error as KeyringError;
+use keyring_core::Error as KeyringError;
 use thiserror::Error;
 
 /// defines the error type for the `KeyStorage`