fix(web): share loopback hostname policy

packages/client/connection/src/loopback-hostname.ts

@@ -0,0 +1,12 @@
+/**
+ * Whether a normalized URL hostname names the local loopback authority.
+ * @param hostname - WHATWG URL hostname (IPv6 literals retain brackets).
+ * @returns true for localhost, IPv6 loopback, or any IPv4 address in 127/8.
+ */
+export function isLoopbackHostname(hostname: string): boolean {
+  if (hostname === 'localhost' || hostname === '[::1]') return true
+  const parts = hostname.split('.')
+  return parts.length === 4
+    && parts[0] === '127'
+    && parts.every(part => /^\d{1,3}$/.test(part) && Number(part) <= 255)
+}