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.
