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

scripts/backup_restore/lib_host_backup_common.sh

@@ -607,6 +607,22 @@ hb_prepare_staging() {
         (( jobs_covered )) || paths+=("$HB_BACKUP_JOBS_DIR")
     fi
 
+    # The logins to private image registries are needed to update the
+    # applications installed from them, whatever the profile selects.
+    local registry_logins=/etc/pve/priv/proxmenux/registry-auth.json
+    if [[ -e "$registry_logins" ]]; then
+        local logins_covered=0 candidate normalized
+        for candidate in "${paths[@]}"; do
+            normalized="${candidate%/}"
+            [[ -z "$normalized" ]] && normalized="/"
+            if [[ "$registry_logins" == "$normalized" || "$registry_logins" == "$normalized"/* ]]; then
+                logins_covered=1
+                break
+            fi
+        done
+        (( logins_covered )) || paths+=("$registry_logins")
+    fi
+
     rm -rf "$staging_root"
     mkdir -p "$staging_root/rootfs" "$staging_root/metadata"