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).
+      // The backend rejects a duplicate name with 409 (Plex conflates same names) — localize it to
-      notifyActionError(e, t("plex.collection.editFailed"));
+      // nameTaken rather than surfacing the server's raw English detail.
+      notifyCollectionEditError(e, t("plex.collection.editFailed"), t("plex.collection.nameTaken"));
     } finally {
       setBusy(null);
     }

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);
     }

frontend/src/components/PlexLibraryPicker.dom.test.tsx

@@ -0,0 +1,101 @@
+// @vitest-environment jsdom
+//
+// Contract of the shared library search-and-pick list (F6): it owns the debounced query and the
+// full loading/error/empty/no-results/list state machine so the two callers (AddItemPanel in
+// PlexCollectionView, NewCollectionPanel in plexBrowseLists) can't drift. The two things a caller
+// steers — the `enabled` gate (NewCollectionPanel holds it off until the collection name is valid)
+// and the `emptyHint` shown before a term is typed — are the pieces with no test before this file, so
+// they're covered here. i18n is key-passthrough and api/debounce are mocked so the test drives state
+// directly.
+import { type ComponentProps } from "react";
+import { describe, it, expect, afterEach, beforeEach, vi } from "vitest";
+import { render, screen, cleanup, waitFor } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+
+vi.mock("react-i18next", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("react-i18next")>();
+  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+});
+// Identity debounce so a passed term is reflected immediately.
+vi.mock("../lib/useDebounced", () => ({ useDebounced: (v: unknown) => v }));
+
+// api.plexLibrary stub — returns whatever the current test queued.
+const plexLibrary = vi.fn();
+vi.mock("../lib/api", () => ({ api: { plexLibrary: (...a: unknown[]) => plexLibrary(...a) } }));
+
+import PlexLibraryPicker from "./PlexLibraryPicker";
+
+const card = (id: string, title: string, year = 2000) => ({
+  id,
+  type: "movie" as const,
+  title,
+  year,
+  thumb: `/api/plex/image/${id}`,
+});
+const result = (items: ReturnType<typeof card>[]) => ({
+  scope: "both",
+  total: items.length,
+  offset: 0,
+  limit: 12,
+  items,
+  episodes: [],
+});
+
+beforeEach(() => {
+  plexLibrary.mockReset();
+  plexLibrary.mockResolvedValue(result([]));
+});
+afterEach(cleanup);
+
+const renderPicker = (props: Partial<ComponentProps<typeof PlexLibraryPicker>> = {}) => {
+  const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  return render(
+    <QueryClientProvider client={qc}>
+      <PlexLibraryPicker
+        scope="both"
+        term=""
+        renderAction={(c) => <button>act:{c.id}</button>}
+        {...props}
+      />
+    </QueryClientProvider>,
+  );
+};
+
+describe("PlexLibraryPicker", () => {
+  it("renders nothing and fires no query while disabled, even with a term", () => {
+    const { container } = renderPicker({ enabled: false, term: "matrix" });
+    expect(container.firstChild).toBeNull();
+    expect(plexLibrary).not.toHaveBeenCalled();
+  });
+
+  it("shows the emptyHint (and fires no query) before a term is typed", () => {
+    renderPicker({ term: "", emptyHint: <p>EMPTY_HINT</p> });
+    expect(screen.getByText("EMPTY_HINT")).toBeTruthy();
+    expect(plexLibrary).not.toHaveBeenCalled();
+  });
+
+  it("queries the library once enabled + a term are both present and renders the rows via renderAction", async () => {
+    plexLibrary.mockResolvedValue(result([card("m1", "The Matrix"), card("m2", "Reloaded")]));
+    renderPicker({ term: "matrix" });
+
+    await waitFor(() => expect(screen.getByText("act:m1")).toBeTruthy());
+    expect(screen.getByText("act:m2")).toBeTruthy();
+    expect(screen.getByText("The Matrix")).toBeTruthy();
+    expect(plexLibrary).toHaveBeenCalledWith({ scope: "both", q: "matrix", limit: 12 });
+  });
+
+  it("renders the honest error state (not an empty/no-results state) when the query fails", async () => {
+    plexLibrary.mockRejectedValue(new Error("boom"));
+    renderPicker({ term: "matrix" });
+
+    await waitFor(() => expect(screen.getByText("common.loadError")).toBeTruthy());
+    expect(screen.queryByText("plex.collection.addNoResults")).toBeNull();
+  });
+
+  it("shows the no-results message when the query returns an empty list", async () => {
+    plexLibrary.mockResolvedValue(result([]));
+    renderPicker({ term: "zzzz" });
+
+    await waitFor(() => expect(screen.getByText("plex.collection.addNoResults")).toBeTruthy());
+  });
+});