fix(playlists): a failed background refetch keeps the list on screen, and the player outlives every branch

Deep-review round 3 of bug hunt 2026-09.

- The detail's error branch applies only when there is nothing to show (`isError && <no data>`, the
  QueryState convention): one failed refetch — on focus, after a move or a removal — replaced a
  loaded list with "couldn't load". The error line is announced (role=alert).
- Tests for the two branches of "the player survives a change of selection" that round 2 left
  untested (a list that fails to load, no selection).

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

@@ -221,6 +221,23 @@ describe("Playlists — removing a video from inside the player", () => {
   });
 });
 
+describe("Playlists — a failed refetch of a list that is on screen", () => {
+  it("keeps the list, instead of replacing it with the error", async () => {
+    // A background refetch — on focus, after a move or a removal — failing during an API restart
+    // replaced a loaded list with "couldn't load" (the `isError && <no data>` convention, ignored).
+    mount({ 1: [vid("V", 0)] });
+    await screen.findByRole("button", { name: "card.play — Video V" });
+    vi.mocked(api.playlist).mockRejectedValue(new HttpError(502, "gateway"));
+    await act(async () => {
+      await qc.invalidateQueries({ queryKey: qk.playlist(1) });
+      await new Promise((r) => setTimeout(r, 20)); // react-query notifies its observers on a timer
+    });
+    expect(qc.getQueryState(qk.playlist(1))?.status).toBe("error");
+    expect(screen.queryByText("playlists.loadFailed")).toBeNull();
+    expect(screen.getByRole("button", { name: "card.play — Video V" })).toBeTruthy();
+  });
+});
+
 describe("Playlists — the player outlives a change of selection", () => {
   it("stays mounted while a newly selected list is still loading", async () => {
     // The page swaps itself for "Loading…" until the new list arrives; with the player rendered only
@@ -235,6 +252,27 @@ describe("Playlists — the player outlives a change of selection", () => {
     expect(h.mounted).toBe(true);
   });
 
+  it("stays mounted when the newly selected list fails to load — a list deleted elsewhere", async () => {
+    const { rerender } = mount({ 1: [vid("V", 0)] });
+    await play("Video V");
+    await waitFor(() => expect(h.mounted).toBe(true));
+    vi.mocked(api.playlist).mockRejectedValue(new HttpError(404, "gone"));
+    h.selectedId = 2;
+    rerender();
+    await screen.findByText("playlists.loadFailed");
+    expect(h.mounted).toBe(true);
+  });
+
+  it("stays mounted when nothing is selected any more", async () => {
+    const { rerender } = mount({ 1: [vid("V", 0)] });
+    await play("Video V");
+    await waitFor(() => expect(h.mounted).toBe(true));
+    h.selectedId = null;
+    rerender();
+    await screen.findByText("playlists.pickOne");
+    expect(h.mounted).toBe(true);
+  });
+
   it("refreshes the list it was launched from when the viewer marks the video in it", async () => {
     // Invalidating the SELECTED list left the launch list's row showing the old status.
     vi.spyOn(api, "setState").mockResolvedValue(undefined as never);

frontend/src/components/Playlists.tsx

@@ -813,11 +813,13 @@ export default function Playlists() {
     );
   }
   // The page used to render "Loading…" forever when the detail query failed — an error read as a
-  // slow network that never arrived.
-  if (detailQuery.isError) {
+  // slow network that never arrived. Only with nothing to show, though (the `isError && <no data>`
+  // convention, see ui/QueryState): a background refetch that failed — on focus, after a move or a
+  // removal — replaced a list that was on screen with this message.
+  if (detailQuery.isError && !detail) {
     return (
       <>
-        <div className="text-sm p-4 flex flex-col items-start gap-3">
+        <div role="alert" className="text-sm p-4 flex flex-col items-start gap-3">
           <span className="inline-flex items-center gap-2 text-muted">
             <AlertTriangle className="w-4 h-4 text-danger" />
             {t("playlists.loadFailed")}