fix(vpn): use 'id' keyword instead of '--' in nmcli connection up/down

Newer nmcli no longer parses '--' as end-of-options for 'connection up',
it treats it as the connection name and fails with "Unbekannte
Verbindung »--«". The 'id' keyword is the canonical way to mark the
following argument as the connection name.

defaults/bin/moonarch-vpn

@@ -39,13 +39,13 @@ function extract_connection_name() {
 # Requires nm-applet (or another NM secret agent) for interactive auth.
 function connect_vpn() {
     local connection="$1"
-    nmcli connection up -- "$connection"
+    nmcli connection up id "$connection"
 }
 
 # Disconnect a VPN.
 function disconnect_vpn() {
     local connection="$1"
-    nmcli connection down -- "$connection"
+    nmcli connection down id "$connection"
 }
 
 # Toggle the VPN connection based on its current state.