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/posts/PostSeriesMenu.vue

@@ -32,11 +32,13 @@
           </p>
           <v-autocomplete
             v-model="picked"
+            v-model:menu="pickerMenuOpen"
             :items="hits"
             :item-title="(s) => s.name"
             :item-value="(s) => s.id"
             :loading="searching"
             label="Series" density="compact" autofocus clearable
+            @keydown.enter.capture="onEnter"
           />
         </v-card-text>
         <v-card-actions>
@@ -56,6 +58,7 @@
 import { ref } from 'vue'
 import { useRouter } from 'vue-router'
 import { useApi } from '../../composables/useApi.js'
+import { useAcceptOnEnter } from '../../composables/useAcceptOnEnter.js'
 import { toast } from '../../utils/toast.js'
 
 const props = defineProps({ post: { type: Object, required: true } })
@@ -70,6 +73,12 @@ const picked = ref(null)
 const hits = ref([])
 const searching = ref(false)
 
+// Enter on the closed dropdown adds the chosen series instead of re-opening it.
+// (aliased — `menuOpen` above is the outer "Series ▾" v-menu's state.)
+const { menuOpen: pickerMenuOpen, onEnter } = useAcceptOnEnter(() => {
+  if (picked.value != null && !busy.value) onAddExisting()
+})
+
 async function onPromote() {
   menuOpen.value = false
   busy.value = true