feat: protect write endpoints with JWT auth (story #08)

Add bcrypt user file auth, JWT middleware on all write routes, Pinia
authStore with localStorage persistence, login view with redirect
support, and v-if guards on all CRUD controls and admin nav link.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/PublicationList.vue

@@ -1,8 +1,10 @@
 <script setup lang="ts">
 import { onMounted } from 'vue'
 import { usePublicationStore } from '../stores/publicationStore'
+import { useAuthStore } from '../stores/authStore'
 
 const store = usePublicationStore()
+const auth = useAuthStore()
 
 onMounted(async () => {
   try {
@@ -18,6 +20,7 @@ onMounted(async () => {
     <div class="mb-4 flex items-center justify-between">
       <h1 class="text-2xl font-bold">Publikationen</h1>
       <router-link
+        v-if="auth.isLoggedIn"
         to="/publications/new"
         class="rounded bg-green-600 px-4 py-2 text-white hover:bg-green-700"
       >