feat(app): host-setup Step 3 — connection form with remote address warning

Renders the address/user/unit form and surfaces a non-blocking warning
when the host address is outside the same-host whitelist.

app/src/views/preference/components/NginxHostSetup/steps/Step3Connection.vue

@@ -0,0 +1,68 @@
+<script setup lang="ts">
+import type { SetupParams } from '@/api/host_setup'
+import { computed, ref, watch } from 'vue'
+
+const params = defineModel<SetupParams>('params', { required: true })
+
+const hostInput = ref(params.value.host_address ?? '')
+watch(hostInput, v => {
+  params.value.host_address = v
+  params.value.use_host_gateway = v.startsWith('host.docker.internal')
+})
+
+const remoteWarning = computed<boolean>(() => {
+  const host = (hostInput.value.split(':')[0] || '').trim()
+  if (!host)
+    return false
+  if (host === 'host.docker.internal')
+    return false
+  if (host === 'localhost' || host === '::1')
+    return false
+  if (/^127\./.test(host))
+    return false
+  if (/^172\.(?:1[6-9]|2\d|3[01])\.0\.1$/.test(host))
+    return false
+  return true
+})
+</script>
+
+<template>
+  <div class="space-y-4">
+    <AFormItem :label="$gettext('Host address (host:port)')" required>
+      <AInput v-model:value="hostInput" placeholder="host.docker.internal:22" />
+    </AFormItem>
+
+    <AFormItem :label="$gettext('SSH user')" required>
+      <AInput v-model:value="params.host_user" placeholder="nginxui" />
+    </AFormItem>
+
+    <AFormItem :label="$gettext('systemd unit')">
+      <AInput v-model:value="params.systemd_unit" placeholder="nginx.service" />
+    </AFormItem>
+
+    <AFormItem :label="$gettext('systemctl path')">
+      <AInput v-model:value="params.systemctl_path" placeholder="/bin/systemctl" />
+    </AFormItem>
+
+    <AAlert
+      v-if="remoteWarning"
+      type="warning"
+      show-icon
+    >
+      <template #message>
+        {{ $gettext('Remote address detected') }}
+      </template>
+      <template #description>
+        {{ $gettext('This mode only supports nginx-ui and target nginx on the same host.') }}
+        {{ $gettext('bind-mount cannot reach remote filesystems — config editing, log viewing and certificate management will fail.') }}
+        <a
+          href="/docs/guide/cluster-node-cross-host.html"
+          target="_blank"
+          rel="noopener"
+        >
+          {{ $gettext('Use the cluster Node guide for cross-host setups.') }}
+        </a>
+      </template>
+    </AAlert>
+  </div>
+</template>