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/components/modal/AliasPickerDialog.vue

@@ -8,6 +8,7 @@
       </p>
       <v-autocomplete
         v-model="selectedId"
+        v-model:menu="menuOpen"
         :items="results"
         :item-title="(t) => t.fandom_name ? `${t.name} — ${t.fandom_name}` : t.name"
         :item-value="(t) => t.id"
@@ -15,6 +16,7 @@
         label="Canonical tag"
         no-filter clearable density="compact"
         @update:search="onSearch"
+        @keydown.enter.capture="onEnter"
       />
     </v-card-text>
     <v-card-actions>
@@ -31,9 +33,10 @@
 <script setup>
 import { ref } from 'vue'
 import { useApi } from '../../composables/useApi.js'
+import { useAcceptOnEnter } from '../../composables/useAcceptOnEnter.js'
 
 const props = defineProps({ category: { type: String, required: true } })
-defineEmits(['confirm', 'cancel'])
+const emit = defineEmits(['confirm', 'cancel'])
 
 const api = useApi()
 const results = ref([])
@@ -41,6 +44,11 @@ const loading = ref(false)
 const selectedId = ref(null)
 let debounce = null
 
+// Enter on the closed dropdown confirms the selection instead of re-opening it.
+const { menuOpen, onEnter } = useAcceptOnEnter(() => {
+  if (selectedId.value != null) emit('confirm', selectedId.value)
+})
+
 function onSearch(q) {
   if (debounce) clearTimeout(debounce)
   debounce = setTimeout(async () => {