feat(qbz-slint): restore saved sessions from the encrypted token store

The MVP no longer requires re-authenticating on every launch. On
startup it loads the saved OAuth token via qbz-credentials, validates
it against Qobuz, and — if valid — activates the session and goes
straight to the Discover/Home view. A rejected token is cleared and
the login screen is shown instead. Successful sign-ins now persist the
token to the shared encrypted store.

- auth.rs: save_oauth_token after login, restore_saved_session at
  startup.
- main.rs: enter_shell helper shared by the login and restore paths.

cc

crates/qbz-slint/Cargo.toml

@@ -24,6 +24,7 @@ qbz-app = { path = "../qbz-app" }
 qbz-core = { path = "../qbz-core" }
 qbz-models = { path = "../qbz-models" }
 qbz-cache = { path = "../qbz-cache" }
+qbz-credentials = { path = "../qbz-credentials" }
 
 [build-dependencies]
 slint-build = "1"