fix(lxc): enable nesting=1 by default for systemd 255+ compat

Ubuntu 24.04 / Debian 13 ship with systemd >= 255 which requires
features=nesting=1 in unprivileged LXC containers. Without it,
container start emits 'WARN: Systemd 255 detected. You may need
to enable nesting.' Now nesting is enabled by default unless the
caller explicitly passes a 'features' string.

backend/app/proxmox_client.py

@@ -2429,8 +2429,12 @@ class ProxmoxClient:
             if description:
                 params['description'] = description
             
+            # Enable nesting by default — required for systemd 255+ (Ubuntu 24.04, Debian 13)
+            # to avoid "WARN: Systemd 255 detected. You may need to enable nesting." warning
             if features:
                 params['features'] = features
+            else:
+                params['features'] = 'nesting=1'
 
             if nameserver:
                 params['nameserver'] = nameserver