fix(settings): a tab-return read never undoes a pick or save made while it was out

- session-ui.js: drop the answer when the case-sync chain moved during the read; settings-ui.js calls _applyServerCaseSettings with ?.
- i18n.js: zh-CN for the failed case-options save toast, case name passed through
- tests: regression for the stale read, zh-CN toast row

test/settings-save-failure-ui.test.ts

@@ -526,6 +526,10 @@ describe('the new toasts in zh-CN', () => {
         'Saved on this device, not on the server: the server could not be reached',
         '已保存到本设备，但未保存到服务器：无法连接到服务器',
       ],
+      [
+        'Options for "demo" (until the next sync) saved on this device, not on the server: HTTP 500',
+        '案例“demo”的选项已保存到本设备（同步前有效），但未保存到服务器：HTTP 500',
+      ],
       [`Tunnel not stopped: ${TORN}`, `隧道未停止：${TORN}`],
     ];
     for (const [english, chinese] of cases) {

test/sync-case-state.test.ts

@@ -262,6 +262,30 @@ describe('returning to the tab', () => {
     expect(unchanged.hidden.size).toBe(0);
   });
 
+  it('does not undo a pick or a case save made while its read was out', async () => {
+    let release!: () => void;
+    const gate = new Promise<void>((r) => (release = r));
+    const { app, select, store } = makeApp({ syncCaseState: true }, async (_url, init) => {
+      if (init?.method === 'PUT') return { body: { success: true, data: {} } };
+      await gate; // the server read this copy before the pick and the save below reached it
+      return env({
+        syncCaseState: true,
+        lastPickedCase: 'alpha',
+        caseSettings: { beta: { agentTeams: false, opusContext1m: true } },
+      });
+    });
+    const refresh = app._syncCaseStateFromServer();
+    await settle();
+    select.value = 'beta';
+    await app.saveLastUsedCase('beta');
+    app.saveCaseSettings('beta', entry);
+    await app._caseSyncChain;
+    release();
+    await refresh;
+    expect(select.value).toBe('beta');
+    expect(JSON.parse(store.get('caseSettings_beta')!)).toEqual(entry);
+  });
+
   it('asks for nothing on a tab return while this device has sync off', async () => {
     const { app, select, calls } = makeApp({}, () => env({ syncCaseState: true, lastUsedCase: 'beta' }));
     await app._syncCaseStateFromServer();