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/PlexCollectionView.tsx

@@ -5,7 +5,7 @@ import { Play, Shuffle, Pencil, Trash2, Plus, X, Lock, Check, Search } from "luc
 import { qk } from "../lib/queryKeys";
 import { api, isIntrinsicReadOnly, type PlexCard, type PlexCollectionDetail } from "../lib/api";
 import { formatRuntime } from "../lib/format";
-import { notifyActionError } from "../lib/actionErrors";
+import { notifyActionError, notifyCollectionEditError } from "../lib/actionErrors";
 import { useConfirm } from "./ConfirmProvider";
 import { BackBtn, ActionBtn, DetailHero } from "../lib/plexDetailUi";
 import { PlexPosterCard } from "./plexCards";
@@ -99,7 +99,8 @@ export default function PlexCollectionView({
       invalidate();
       setRenaming(false);
     } catch (e) {
-      notifyActionError(e, t("plex.collection.editFailed"));
+      // The backend enforces per-library name uniqueness — localize its 409 to nameTaken.
+      notifyCollectionEditError(e, t("plex.collection.editFailed"), t("plex.collection.nameTaken"));
     } finally {
       setBusy(false);
     }