backups: fix the history view's back link

The view linked to /assets/<assetid>, which is not a route. Each asset type
has its own detail route keyed by its own id (/machines/<machineid>), and
there is no generic asset detail page to return to, so the link 404'd.

Uses router.back() instead: this page is only ever reached from an asset
panel, so history is the correct destination regardless of the asset type,
and it needs no per-type route table. Falls back to the machines list when
opened directly from a pasted URL.

The /api/assets/<id> call that fetches the heading name is unaffected - that
endpoint does exist; it was only the frontend route that did not.

plugins/backups/frontend/views/BackupHistory.vue

@@ -1,7 +1,11 @@
 <template>
   <div class="bh">
     <div class="bh-head">
-      <router-link :to="`/assets/${assetid}`" class="bh-back">&larr; Back to asset</router-link>
+      <!-- back(), not a route: this page is reached from an asset panel, and
+           each asset type has its own detail route keyed by its OWN id
+           (/machines/:machineid, not the assetid we are given). There is no
+           generic /assets/:id to link to. -->
+      <button type="button" class="bh-back" @click="goback">&larr; Back</button>
       <h1>Configuration Backups</h1>
       <p v-if="assetname" class="bh-sub">{{ assetname }}</p>
     </div>
@@ -90,12 +94,18 @@
 
 <script setup>
 import { ref, onMounted } from 'vue'
-import { useRoute } from 'vue-router'
+import { useRoute, useRouter } from 'vue-router'
 import api from '@/api'
 
 const route = useRoute()
+const router = useRouter()
 const assetid = route.params.assetid
 
+function goback() {
+  if (window.history.length > 1) router.back()
+  else router.push('/machines')
+}
+
 const revisions = ref([])
 const diffs = ref({})
 const assetname = ref('')
@@ -177,7 +187,10 @@ onMounted(load)
 <style scoped>
 .bh { padding: 1.5rem; }
 .bh-head { margin-bottom: 1rem; }
-.bh-back { font-size: 0.85rem; }
+.bh-back {
+  font-size: 0.85rem; background: none; border: none; padding: 0;
+  color: #0d6efd; cursor: pointer;
+}
 .bh-sub { color: #666; margin: 0.2rem 0 0; }
 .bh-note, .bh-muted { color: #666; }
 .bh-error { color: #c00; }