feat(ssh): probe none authentication before credentials

Some SSH proxies and jump-hosts authenticate with the "none" method,
requiring no password or key. dbx only tried publickey/password/agent,
so connecting through such a host failed with "SSH public key
authentication failed" — and the frontend forced users to enter a
credential just to pass validation.

- Backend: probe authenticate_none first in connect_and_authenticate;
  on rejection fall back to the configured credential method, matching
  standard openssh behavior.
- Frontend: drop the "password/key/agent required" check in
  validateTransportLayers so passwordless hops can be saved and tested.

Refs #1628

apps/desktop/src/components/connection/ConnectionDialog.vue

@@ -2139,9 +2139,9 @@ function validateTransportLayers(config: LegacyConnectionConfig) {
     }
     if (layer.type === "ssh") {
       if (!layer.user?.trim()) throw new Error(t("connection.sshHopInvalidUser", { hop: label }));
-      if (!layer.password?.trim() && !layer.key_path?.trim() && !layer.use_ssh_agent) {
-        throw new Error(t("connection.sshHopInvalidAuth", { hop: label }));
-      }
+      // Auth credentials are optional: the backend probes "none" authentication
+      // first, so hops that require no credential (e.g. passwordless SSH proxies)
+      // are valid with password, key, and agent all left empty.
       const timeout = Number(layer.connect_timeout_secs);
       if (!Number.isFinite(timeout) || timeout < 1 || timeout > 300) {
         throw new Error(t("connection.sshHopInvalidTimeout", { hop: label }));