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/components/BackLink.vue

@@ -0,0 +1,28 @@
+<template>
+  <RouterLink to="/" class="back-link" aria-label="Back to home">
+    <svg width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+      <polyline points="15 18 9 12 15 6" />
+    </svg>
+    <span class="back-link__brand">fete</span>
+  </RouterLink>
+</template>
+
+<script setup lang="ts">
+import { RouterLink } from 'vue-router'
+</script>
+
+<style scoped>
+.back-link {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.15rem;
+  color: var(--color-text-on-gradient);
+  text-decoration: none;
+  line-height: 1;
+}
+
+.back-link__brand {
+  font-size: 1.3rem;
+  font-weight: 700;
+}
+</style>