fix(web): share loopback hostname policy

packages/client/connection/tests/loopback-hostname.spec.ts

@@ -0,0 +1,18 @@
+/** Shared loopback-hostname semantics for the Host fence and browser UI. */
+
+import { describe, expect, it } from 'vitest'
+import { isLoopbackHostname } from '../src/loopback-hostname.ts'
+
+describe('isLoopbackHostname', () => {
+  it('accepts localhost, IPv6 loopback, and the whole IPv4 127/8 block', () => {
+    for (const hostname of ['localhost', '[::1]', '127.0.0.1', '127.8.9.10', '127.255.255.255']) {
+      expect(isLoopbackHostname(hostname)).toBe(true)
+    }
+  })
+
+  it('refuses malformed and non-loopback hostnames', () => {
+    for (const hostname of ['remote.localhost', '::1', '128.0.0.1', '127.0.0', '127.0.0.256', '127.0.0.-1']) {
+      expect(isLoopbackHostname(hostname)).toBe(false)
+    }
+  })
+})