refactor(gateway): share IPv4 input validator

src/shared/net/ipv4.ts

@@ -0,0 +1,19 @@
+export function validateIPv4AddressInput(value: string | undefined): string | undefined {
+  if (!value) {
+    return "IP address is required for custom bind mode";
+  }
+  const trimmed = value.trim();
+  const parts = trimmed.split(".");
+  if (parts.length !== 4) {
+    return "Invalid IPv4 address (e.g., 192.168.1.100)";
+  }
+  if (
+    parts.every((part) => {
+      const n = parseInt(part, 10);
+      return !Number.isNaN(n) && n >= 0 && n <= 255 && part === String(n);
+    })
+  ) {
+    return undefined;
+  }
+  return "Invalid IPv4 address (each octet must be 0-255)";
+}