Fix VPN DNS resolution in DHCP mode (#1514)

Empty DNS= and FallbackDNS= in resolved.conf were blocking VPN DNS
servers when VPN clients replaced /etc/resolv.conf.

Removing these empty values allows systemd-resolved to:
- Use DHCP DNS from network interfaces (existing behavior)
- Fall back to system defaults when needed
- Switch to 'foreign' mode for VPN DNS override

Fixes #1509

bin/omarchy-setup-dns

@@ -37,8 +37,6 @@ EOF
 DHCP)
   sudo tee /etc/systemd/resolved.conf >/dev/null <<'EOF'
 [Resolve]
-DNS=
-FallbackDNS=
 DNSOverTLS=no
 EOF