refactor: extract back button into shared component using history navigation

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

src/components/BackButton.vue

@@ -0,0 +1,28 @@
+<script setup lang="ts">
+import { useRouter } from "vue-router"
+
+const router = useRouter()
+const goBack = () => router.back()
+</script>
+
+<template>
+  <button class="button is-small is-white back-button" @click="goBack">
+    <svg
+      xmlns="http://www.w3.org/2000/svg"
+      class="icon icon-tabler icon-tabler-arrow-narrow-left"
+      width="28"
+      height="28"
+      viewBox="0 0 24 24"
+      stroke-width="1.5"
+      stroke="currentColor"
+      fill="none"
+      stroke-linecap="round"
+      stroke-linejoin="round"
+    >
+      <line x1="5" y1="12" x2="19" y2="12" />
+      <line x1="5" y1="12" x2="9" y2="16" />
+      <line x1="5" y1="12" x2="9" y2="8" />
+    </svg>
+    return
+  </button>
+</template>