fix: integrate updater downgrade guard

tests/unit/app-updater.test.ts

@@ -11,11 +11,19 @@ const loggerMocks = vi.hoisted(() => ({
 
 const updaterMocks = vi.hoisted(() => {
   const listeners = new Map<string, Array<(...args: unknown[]) => void>>();
+  let channel = 'latest';
   const autoUpdater = {
     autoDownload: false,
     autoInstallOnAppQuit: true,
+    allowDowngrade: false,
     logger: null as unknown,
-    channel: 'latest',
+    get channel() {
+      return channel;
+    },
+    set channel(value: string) {
+      channel = value;
+      autoUpdater.allowDowngrade = true;
+    },
     on: vi.fn((event: string, listener: (...args: unknown[]) => void) => {
       const current = listeners.get(event) ?? [];
       current.push(listener);
@@ -95,6 +103,7 @@ describe('AppUpdater feed delegation', () => {
     vi.clearAllMocks();
     electronMocks.getVersion.mockReturnValue('0.9.1');
     updaterMocks.autoUpdater.channel = 'latest';
+    updaterMocks.autoUpdater.allowDowngrade = false;
     updaterMocks.autoUpdater.checkForUpdates.mockResolvedValue(null);
   });
 
@@ -131,6 +140,28 @@ describe('AppUpdater feed delegation', () => {
     }
   });
 
+  it('ignores an older stable feed version even when electron-updater reports it as available', async () => {
+    electronMocks.getVersion.mockReturnValue('2.0.0');
+    updaterMocks.autoUpdater.checkForUpdates.mockImplementation(async () => ({
+      isUpdateAvailable: updaterMocks.autoUpdater.allowDowngrade,
+      updateInfo: { version: '1.1.9' },
+    }));
+    const updater = new AppUpdater();
+
+    await updater.checkForUpdates();
+
+    expect(updaterMocks.autoUpdater.allowDowngrade).toBe(false);
+    expect(updater.getStatus()).toMatchObject({ status: 'not-available' });
+  });
+
+  it('keeps downgrade disabled when changing update channels', () => {
+    const updater = new AppUpdater();
+
+    updater.setChannel('beta');
+
+    expect(updaterMocks.autoUpdater.allowDowngrade).toBe(false);
+  });
+
   it('reports a concise actionable error when the stable feed has no latest manifest', async () => {
     const rawError = new Error(
       'Cannot find channel "latest.yml" update info: HttpError: 404\n' +