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/FandomSetDialog.vue

@@ -82,6 +82,7 @@
 <script setup>
 import { nextTick, onMounted, ref } from 'vue'
 import { useTagStore } from '../../stores/tags.js'
+import { useAcceptOnEnter } from '../../composables/useAcceptOnEnter.js'
 
 const props = defineProps({ tag: { type: Object, required: true } })
 const emit = defineEmits(['updated', 'cancel'])
@@ -93,10 +94,15 @@ const newName = ref('')
 const busy = ref(false)
 const error = ref(null)
 const collision = ref(null)
-const menuOpen = ref(false)
 const fandomRef = ref(null)
 const newNameRef = ref(null)
 
+// Enter on the closed dropdown Saves the changed fandom instead of re-opening it.
+const { menuOpen, onEnter: onSearchEnter } = useAcceptOnEnter(() => {
+  if (busy.value) return
+  if (selectedId.value !== (props.tag.fandom_id ?? null)) onSave()
+})
+
 // Always refresh on open so the list reflects fandoms created elsewhere (#712).
 onMounted(() => store.loadFandoms())
 
@@ -131,20 +137,6 @@ async function onCreate() {
   }
 }
 
-// Enter on the search field — bound in the CAPTURE phase so it runs BEFORE
-// Vuetify's own handler (which opens the menu on Enter). Menu open → let Vuetify
-// pick the highlighted item. Menu closed with a changed selection → Save and stop
-// the event so Vuetify never (re)opens the dropdown (the bug this fixes).
-function onSearchEnter(e) {
-  if (menuOpen.value) return
-  if (busy.value) return
-  if (selectedId.value !== (props.tag.fandom_id ?? null)) {
-    e.preventDefault()
-    e.stopPropagation()
-    onSave()
-  }
-}
-
 // Tab from the search field goes to the "new fandom" text field (operator-
 // specified). Shift+Tab keeps normal reverse traversal.
 function onSearchTab(e) {