Implement code word based remote authentication (#1)

Add comprehensive remote authentication system allowing users to log in from one device by authenticating from another trusted device. Features include:

- Proof of Work (PoW) protection using PBKDF2-SHA512 to prevent abuse
- Simple pairing codes (3 words) protected by dynamic PoW difficulty
- Autocomplete pairing code input with error checking
- Real-time WebSocket communication between devices

Unlike device addition links and reset links with QR codes that only allow adding an authentication method, and that work offline over the duration of several days, this mechanism is strictly online, with 5 minute time limit.

frontend/auth/admin/AdminApp.vue

@@ -3,7 +3,6 @@ import { ref, onMounted, onUnmounted, computed, watch } from 'vue'
 import Breadcrumbs from '@/components/Breadcrumbs.vue'
 import CredentialList from '@/components/CredentialList.vue'
 import UserBasicInfo from '@/components/UserBasicInfo.vue'
-import RegistrationLinkModal from '@/components/RegistrationLinkModal.vue'
 import StatusMessage from '@/components/StatusMessage.vue'
 import LoadingView from '@/components/LoadingView.vue'
 import AuthRequiredMessage from '@/components/AccessDenied.vue'

frontend/auth/restricted/RestrictedApi.vue

@@ -1,15 +1,35 @@
 <template>
   <RestrictedAuth
     :mode="authMode"
+    :remote-auth-token="remoteAuthToken"
     @authenticated="handleAuthenticated"
     @back="handleBack"
   />
 </template>
 
 <script setup>
-import { computed, onMounted } from 'vue'
+import { computed, onMounted, ref } from 'vue'
 import RestrictedAuth from '@/components/RestrictedAuth.vue'
 
+// Check if this is a remote auth URL: /auth/{token}
+// The token is a 5-word passphrase like "word1.word2.word3.word4.word5"
+const remoteAuthToken = ref(null)
+
+function extractRemoteToken() {
+  const path = window.location.pathname
+  // Match /auth/{token} where token is a passphrase with dots
+  const match = path.match(/\/auth\/([^/]+)$/)
+  if (match) {
+    const token = match[1]
+    // Validate it looks like a 5-word passphrase
+    const parts = token.split('.')
+    if (parts.length === 5 && parts.every(p => p.length > 0)) {
+      return token
+    }
+  }
+  return null
+}
+
 // Detect mode from URL hash fragment
 const authMode = computed(() => {
   const params = new URLSearchParams(window.location.hash.slice(1))
@@ -40,6 +60,9 @@ function handleBack() {
 }
 
 onMounted(() => {
+  // Check for remote auth token in URL
+  remoteAuthToken.value = extractRemoteToken()
+
   postToParent({
     type: 'auth-ready'
   })