Add the ProtonVPN server networks to the IP lookup (closes #236)

Adds the VPN list `protonvpn`, downloaded once a day. It shows in `flags.is_vpn` and `vpn_providers` with no other change.

Proton's `/vpn/logicals` answers 401 without a logged-in session, even with a current client's `x-pm-appversion`, so the list comes from the public ProtonVPN-IPs mirror. No request header is needed.

Judgement call: the mirror's /24 file, not its exact addresses. Its README says traffic leaves from unpublished addresses in those networks. The cost is that neighbouring addresses are listed too.

Model: opus-5-5

internal/iplist/parse.go

@@ -58,6 +58,8 @@ func parse(r io.Reader, format Format) (*netipx.IPSet, error) {
 		return parsePIA(r)
 	case FormatIVPNJSON:
 		return parseIVPN(r)
+	case FormatProtonVPN:
+		return parseProtonVPN(r)
 	default:
 		return nil, errUnknownFormat
 	}
@@ -329,6 +331,21 @@ func parseIVPN(r io.Reader) (*netipx.IPSet, error) {
 	return addressSet(texts)
 }
 
+// parseProtonVPN returns the networks in a ProtonVPN-IPs list. A file with no
+// networks fails, because the service always has servers.
+func parseProtonVPN(r io.Reader) (*netipx.IPSet, error) {
+	addresses, err := parseLines(r, addAddressOrNetwork)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(addresses.Ranges()) == 0 {
+		return nil, errNoAddresses
+	}
+
+	return addresses, nil
+}
+
 // readJSON reads r to its end and decodes it, as one JSON document, into v.
 func readJSON(r io.Reader, v any) error {
 	data, err := io.ReadAll(r)