test(settings): a late failure of the previous language keeps the history (#372)

Review 25424, point 1: the `if (!cancelled)` of ChangelogContent's
.catch had no test (V1). Modelled on the late-answer test: the French
file rejects after the English one was read, and the history still shows
the English notes, not the empty message.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/components/settings/ChangelogContent.test.tsx

@@ -335,4 +335,29 @@ describe("ChangelogContent — language switch", () => {
     expect(html).toContain("note-0171-en");
     expect(html).not.toMatch(/note-0171(?!-en)/);
   });
+
+  it("never lets a late failure of the previous language empty the current history", async () => {
+    const pending: Array<{ url: string; resolve: (text: string) => void; reject: (error: Error) => void }> = [];
+    fetchMock.mockImplementation(
+      (url: string) =>
+        new Promise<FetchedFile>((resolveFetch, rejectFetch) => {
+          pending.push({ url, resolve: (text) => resolveFetch({ text: async () => text }), reject: rejectFetch });
+        }),
+    );
+
+    render("0.17.1");
+    harness.commit(); // asks for the French file
+    ctx.language = "en";
+    render("0.17.1");
+    harness.commit(); // cancels the French one, asks for the English file
+    expect(pending.map((p) => p.url)).toEqual(["/CHANGELOG.fr.md", "/CHANGELOG.md"]);
+
+    pending[1].resolve(EN);
+    await settle();
+    pending[0].reject(new Error("the French file could not be read"));
+    await settle();
+    const html = renderToStaticMarkup(render("0.17.1"));
+    expect(html).toContain("note-0171-en");
+    expect(html).not.toContain("t(changelog.empty)");
+  });
 });