fix(plex): complete the 409->nameTaken sweep + cover PlexLibraryPicker

The notifyCollectionEditError helper localizes the backend's duplicate-name
409 to plex.collection.nameTaken; it was applied to the New-collection panel
but two sibling collection-write sites still routed 409 through
notifyActionError, surfacing the server's raw English detail under the HU UI:

- PlexCollectionView.doRename (rename collision)
- PlexCollectionEditor.createNew (create-in-editor duplicate name)

Both now use notifyCollectionEditError, matching the create path. Also adds a
direct dom test for the extracted PlexLibraryPicker covering the enabled gate,
emptyHint, results, honest error, and no-results branches.

frontend/src/components/PlexCollectionEditor.tsx

@@ -4,7 +4,7 @@ import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { Check, Plus } from "lucide-react";
 import { api } from "../lib/api";
-import { notifyActionError } from "../lib/actionErrors";
+import { notifyCollectionEditError } from "../lib/actionErrors";
 import { useScrollFade } from "../lib/useScrollFade";
 import Modal from "./Modal";
 import { useConfirm } from "./ConfirmProvider";
@@ -102,8 +102,9 @@ export default function PlexCollectionEditor({
       setMembers((prev) => new Set(prev).add(col.id));
       refresh();
     } catch (e) {
-      // Surfaces the 409 when the name duplicates an existing collection (Plex conflates same names).
-      notifyActionError(e, t("plex.collection.editFailed"));
+      // The backend rejects a duplicate name with 409 (Plex conflates same names) — localize it to
+      // nameTaken rather than surfacing the server's raw English detail.
+      notifyCollectionEditError(e, t("plex.collection.editFailed"), t("plex.collection.nameTaken"));
     } finally {
       setBusy(null);
     }