keep registry accounts for the whole cluster and use them everywhere

- Registry accounts are kept in /etc/pve/priv/proxmenux/registry-auth.json, shared by every node, unless REGISTRY_AUTH_FILE says otherwise
- The custom image importer offers to sign in when a registry refuses an image, and reads it again
- Installers, updates, the watchdog and the Monitor update check read private images with the saved accounts
- The host backup includes the accounts, and uninstalling ProxMenux removes them and keeps the OCI records of the cluster
- Spanish written by hand and spacing fixed for the registry account messages in every locale
- Docs: private registries section, Registry accounts in the OCI menu, README storage notes

.github/scripts/tests/test_oci_wording.py

@@ -124,7 +124,8 @@ class OciWordingTests(TestCase):
         fn = function(CUSTOM, 'explore', read_definition=lambda _: ('definition', 'source'),
             ignored_settings=lambda _: [], template_from_compose=lambda _: {
                 'container_contract': {'image': {'reference': 'example:latest'}}},
-            describe=lambda _: 'SUMMARY', registry_report=lambda _: (False, 'INSPECTION FAILED'))
+            describe=lambda _: 'SUMMARY', registry_report=lambda _: (False, 'INSPECTION FAILED'),
+            _registry_refused=lambda _: False)
         fn(ui)
         self.assertEqual(ui.message.call_args.args[0], 'SUMMARY\n\nINSPECTION FAILED\n\n' + REGISTRY_ADVICE)
         self.assertNotIn('not supported yet', ui.message.call_args.args[0])