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/lib/actionErrors.ts

@@ -12,3 +12,22 @@ export function notifyActionError(err: unknown, fallbackMessage: string): void {
   const detail = err instanceof HttpError ? err.detail : null;
   notify({ level: "error", message: detail || fallbackMessage });
 }
+
+/**
+ * Collection write errors. The backend enforces per-library name uniqueness and returns 409 on a
+ * duplicate; localize that specific case to the friendly `nameTaken` copy (notifyActionError would
+ * otherwise surface the server's raw English detail). Everything else falls back to the generic
+ * edit-failed handling (which still prefers the server's reason when present). Both args are already
+ * translated so this stays i18n-agnostic and unit-testable.
+ */
+export function notifyCollectionEditError(
+  err: unknown,
+  editFailedMessage: string,
+  nameTakenMessage: string,
+): void {
+  if (err instanceof HttpError && err.status === 409) {
+    notify({ level: "error", message: nameTakenMessage });
+    return;
+  }
+  notifyActionError(err, editFailedMessage);
+}