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/composables/useAcceptOnEnter.js

@@ -0,0 +1,35 @@
+import { ref } from 'vue'
+
+/**
+ * Unifies keyboard behavior for confirm-style `v-autocomplete` / `v-select`
+ * dropdowns — a single picker whose Enter should trigger the surrounding
+ * modal's primary action (Merge, Add, Save, Use…).
+ *
+ * The recurring bug (operator-flagged across the merge / fandom / series / alias
+ * dialogs): pressing Enter with the menu CLOSED re-opens the dropdown instead of
+ * accepting the chosen value, because Vuetify's own Enter handler opens the menu.
+ * We bind in the CAPTURE phase so this runs BEFORE Vuetify's handler:
+ *   - menu OPEN   → return; let Vuetify pick the highlighted item (it sets the
+ *                   value and closes the menu).
+ *   - menu CLOSED → preventDefault + stopPropagation so Vuetify can't re-open the
+ *                   menu, then call `accept()` — the modal's confirm. `accept`
+ *                   should itself no-op when nothing valid is selected.
+ *
+ * Usage:
+ *   const { menuOpen, onEnter } = useAcceptOnEnter(onConfirm)
+ *   <v-autocomplete v-model:menu="menuOpen" @keydown.enter.capture="onEnter" … />
+ *
+ * `menuOpen` is exposed so a component that programmatically focuses the field
+ * can force the menu shut (so the very next Enter accepts rather than being
+ * swallowed as a menu interaction).
+ */
+export function useAcceptOnEnter(accept) {
+  const menuOpen = ref(false)
+  function onEnter(e) {
+    if (menuOpen.value) return
+    e.preventDefault()
+    e.stopPropagation()
+    accept()
+  }
+  return { menuOpen, onEnter }
+}