fix(core,cli): end the session on Proton when signing out

Logout only deleted the keyring entry, so the session's tokens stayed
valid on Proton's side. It now ends the session first (DELETE auth/v4,
bounded to 10 s) and signs out locally either way. `pdfs logout` says
when Proton could not be reached (B233).

crates/pdfs-gui/src/app/pages/login.rs

@@ -602,7 +602,7 @@ pub(crate) fn sign_out(ui: &Rc<Ui>) {
             glib::spawn_future_local(async move {
                 let result = gio::spawn_blocking(|| {
                     service::disable_stop();
-                    auth::logout().map_err(|e| e.to_string())
+                    auth::logout().map(|_| ()).map_err(|e| e.to_string())
                 })
                 .await;
                 if let Ok(Err(e)) = result {