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/App.vue

@@ -1,9 +1,26 @@
 <template>
   <div class="app-container">
+    <header v-if="route.name !== 'home'" class="app-header">
+      <BackLink />
+    </header>
     <RouterView />
   </div>
 </template>
 
 <script setup lang="ts">
-import { RouterView } from 'vue-router'
+import { RouterView, useRoute } from 'vue-router'
+import BackLink from '@/components/BackLink.vue'
+
+const route = useRoute()
 </script>
+
+<style scoped>
+.app-header {
+  position: absolute;
+  top: 0;
+  left: 0;
+  right: 0;
+  z-index: 10;
+  padding-top: var(--spacing-lg);
+}
+</style>