test(plex): cover create write-through + the 409->nameTaken mapping (F-F)

Backend: create-collection happy path (write-through + editable) and the
create resync-failure 422 (honest reconcile detail), mirroring the rename/remove
tests. Frontend: extract notifyCollectionEditError (the 409->nameTaken vs
generic-fallback mapping that was inline in the New-collection create()) and
unit-test it; wire create() through it.

frontend/src/components/plexBrowseLists.tsx

@@ -4,16 +4,9 @@ import type { TFunction } from "i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { Check, ListMusic, Lock, Plus, Search, X } from "lucide-react";
 import { qk } from "../lib/queryKeys";
-import {
-  api,
-  HttpError,
-  isIntrinsicReadOnly,
-  type PlexCard,
-  type PlexCollection,
-} from "../lib/api";
+import { api, isIntrinsicReadOnly, type PlexCard, type PlexCollection } from "../lib/api";
 import { formatRuntime } from "../lib/format";
-import { notifyActionError } from "../lib/actionErrors";
-import { notify } from "../lib/notifications";
+import { notifyCollectionEditError } from "../lib/actionErrors";
 import { useDebounced } from "../lib/useDebounced";
 import { LoadingState, StateMessage } from "./QueryState";
 import PosterTile from "./PosterTile";
@@ -102,13 +95,12 @@ function NewCollectionPanel({
       const col = await api.plexCreateCollection("", trimmed, card.id); // backend derives the library
       onCreated(col.id);
     } catch (e) {
-      // The backend is authoritative on per-library name uniqueness — localize its 409 to nameTaken
-      // (notifyActionError would otherwise surface the server's raw English detail).
-      if (e instanceof HttpError && e.status === 409) {
-        notify({ level: "error", message: t("plex.collection.nameTaken") });
-      } else {
-        notifyActionError(e, t("plex.collection.editFailed"));
-      }
+      // The backend is authoritative on per-library name uniqueness — its 409 is localized to nameTaken.
+      notifyCollectionEditError(
+        e,
+        t("plex.collection.editFailed"),
+        t("plex.collection.nameTaken"),
+      );
       setBusy(false);
     }
   }