Extract BackLink component into App layout

Move back navigation (chevron + "fete" brand) from per-view
definitions into a shared BackLink component rendered in App.vue.
Shown on all pages except home. Hero overlay gets pointer-events:
none so the link stays clickable on the event detail page.

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

frontend/src/views/EventDetailView.vue

@@ -8,10 +8,6 @@
         alt=""
       />
       <div class="detail__hero-overlay" />
-      <header class="detail__header">
-        <RouterLink to="/" class="detail__back" aria-label="Back to home">&larr;</RouterLink>
-        <span class="detail__brand">fete</span>
-      </header>
     </div>
 
     <div class="detail__body">
@@ -168,7 +164,7 @@
 
 <script setup lang="ts">
 import { ref, computed, onMounted } from 'vue'
-import { RouterLink, useRoute } from 'vue-router'
+import { useRoute } from 'vue-router'
 import { api } from '@/api/client'
 import { useEventStorage } from '@/composables/useEventStorage'
 import AttendeeList from '@/components/AttendeeList.vue'
@@ -437,32 +433,7 @@ onMounted(fetchEvent)
     var(--color-glass-overlay) 0%,
     transparent 50%
   );
-}
-
-.detail__header {
-  position: absolute;
-  top: 0;
-  left: 0;
-  right: 0;
-  display: flex;
-  align-items: center;
-  gap: var(--spacing-sm);
-  padding: var(--spacing-lg) var(--content-padding);
-  padding-top: env(safe-area-inset-top, var(--spacing-lg));
-  z-index: 1;
-}
-
-.detail__back {
-  color: var(--color-text-on-gradient);
-  font-size: 1.5rem;
-  text-decoration: none;
-  line-height: 1;
-}
-
-.detail__brand {
-  font-size: 1.3rem;
-  font-weight: 700;
-  color: var(--color-text-on-gradient);
+  pointer-events: none;
 }
 
 .detail__body {