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/EventCreateView.vue

@@ -1,9 +1,6 @@
 <template>
   <main class="create">
-    <header class="create__header">
-      <RouterLink to="/" class="create__back" aria-label="Back to home">&larr;</RouterLink>
-      <h1 class="create__title">Create</h1>
-    </header>
+    <h1 class="create__title">Create</h1>
 
     <form class="create__form" novalidate @submit.prevent="handleSubmit">
       <div class="form-group">
@@ -76,7 +73,7 @@
 
 <script setup lang="ts">
 import { reactive, ref, watch } from 'vue'
-import { RouterLink, useRouter } from 'vue-router'
+import { useRouter } from 'vue-router'
 import { api } from '@/api/client'
 import { useEventStorage } from '@/composables/useEventStorage'
 
@@ -194,20 +191,7 @@ async function handleSubmit() {
   display: flex;
   flex-direction: column;
   gap: var(--spacing-lg);
-  padding-top: var(--spacing-lg);
-}
-
-.create__header {
-  display: flex;
-  align-items: center;
-  gap: var(--spacing-sm);
-}
-
-.create__back {
-  color: var(--color-text-on-gradient);
-  font-size: 1.5rem;
-  text-decoration: none;
-  line-height: 1;
+  padding-top: calc(var(--spacing-lg) + 2.5rem);
 }
 
 .create__title {