refactor(ui): unify confirm-dropdown Enter behavior via useAcceptOnEnter

Operator-flagged (again) on the tag-merge picker: Enter on the dropdown re-opens
it instead of accepting the selection. I'd already patched this twice (fandom
picker + fandom set dialog) with copy-pasted capture-phase handlers, so DRY it.

New composable useAcceptOnEnter(accept): tracks the menu state and, on a
capture-phase Enter, lets Vuetify pick when the menu is open but calls accept()
(and blocks the re-open) when it's closed. Applied to every confirm-style picker:
- TagsView merge-into picker (the reported one)
- AliasPickerDialog
- PostSeriesMenu add-to-existing
- FandomPicker + FandomSetDialog (refactored off their bespoke handlers)

One behavior, one place to change it.

frontend/src/views/TagsView.vue

@@ -55,11 +55,13 @@
           </p>
           <v-autocomplete
             v-model="mergeTargetId"
+            v-model:menu="mergeMenuOpen"
             :items="mergeHits" item-title="name" item-value="id"
             :loading="mergeLoading"
             density="compact" variant="outlined" hide-details
             placeholder="Search target tag…" no-filter
             @update:search="onMergeSearch"
+            @keydown.enter.capture="onMergeEnter"
           />
         </v-card-text>
         <v-card-actions>
@@ -107,6 +109,7 @@ import { useTagDirectoryStore } from '../stores/tagDirectory.js'
 import { useAdminStore } from '../stores/admin.js'
 import { useApi } from '../composables/useApi.js'
 import { useInfiniteScroll } from '../composables/useInfiniteScroll.js'
+import { useAcceptOnEnter } from '../composables/useAcceptOnEnter.js'
 import TagCard from '../components/discovery/TagCard.vue'
 import MergeConfirmDialog from '../components/discovery/MergeConfirmDialog.vue'
 import DestructiveConfirmModal from '../components/modal/DestructiveConfirmModal.vue'
@@ -181,6 +184,10 @@ const api = useApi()
 
 // Tag merge via dots-menu (separate from inline-rename collision flow above)
 const mergePickerOpen = ref(false)
+// Enter on the closed dropdown accepts the target instead of re-opening it.
+const { menuOpen: mergeMenuOpen, onEnter: onMergeEnter } = useAcceptOnEnter(
+  () => onMergeConfirm()
+)
 const mergeSource = ref(null)
 const mergeTargetId = ref(null)
 const mergeHits = ref([])