feat(updater): electron-updater wiring + IPC bridge

electron-updater integration so users get fixes automatically:
  - new dependency electron-updater@^6.6.2 in package.json
  - electron-builder.config.js publishes to GitHub releases
    (owner: concreteinfo, repo: markdownconverter)
  - new src/main/auto-updater.js wraps autoUpdater: wires the six
    lifecycle events to a 'updates:status' IPC channel, exposes
    check() and install() helpers, dev-gates via isDev(), and
    swallows the 'missing app-update.yml' error that dev runs hit
  - main.js: ensureAutoUpdater() forwards status to the renderer
    via mainWindow.webContents.send; ipcMain handlers for
    'updates:check' (invoke) and 'updates:install' (invoke)
  - preload.js: 'updates:check' / 'updates:install' added to
    ALLOWED_SEND_CHANNELS; 'updates:status' added to receive
    channels; electronAPI.updates.{check, install, onStatus}
    exposed

7 new tests cover event registration, dev-gate behaviour,
'not-available' fallback for missing app-update.yml, typed event
payloads, error forwarding, and install() quitAndInstall.

Amit Haridas

tests/auto-updater.test.js

@@ -0,0 +1,114 @@
+/**
+ * @jest-environment node
+ *
+ * Auto-updater — unit test the controller without bringing up electron.
+ * The electron-updater module is mocked because the real one requires the
+ * Electron app to be initialised.
+ */
+
+const mockAutoUpdater = {
+  on: jest.fn(),
+  autoDownload: false,
+  autoInstallOnAppQuit: false,
+  checkForUpdates: jest.fn().mockResolvedValue(null),
+  quitAndInstall: jest.fn(),
+};
+
+jest.mock('electron-updater', () => ({ autoUpdater: mockAutoUpdater }), { virtual: true });
+
+const { setupAutoUpdater } = require('../src/main/auto-updater');
+
+describe('setupAutoUpdater', () => {
+  let send;
+  let updater;
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+    mockAutoUpdater.on.mockReset();
+    send = jest.fn();
+    updater = setupAutoUpdater({ send });
+  });
+
+  test('registers all six event handlers', () => {
+    const events = mockAutoUpdater.on.mock.calls.map((c) => c[0]);
+    expect(events).toEqual(
+      expect.arrayContaining([
+        'checking-for-update',
+        'update-available',
+        'update-not-available',
+        'download-progress',
+        'update-downloaded',
+        'error',
+      ])
+    );
+  });
+
+  test('enables autoDownload and autoInstallOnAppQuit', () => {
+    expect(mockAutoUpdater.autoDownload).toBe(true);
+    expect(mockAutoUpdater.autoInstallOnAppQuit).toBe(true);
+  });
+
+  test('check() forwards "not-available" when app-update.yml is missing (dev)', async () => {
+    mockAutoUpdater.checkForUpdates.mockRejectedValueOnce(
+      new Error('Cannot find any update manifest (app-update.yml) in resources')
+    );
+    await updater.check();
+    expect(send).toHaveBeenCalledWith('updates:status', { state: 'not-available' });
+  });
+
+  test('check() forwards "error" for non-benign failures', async () => {
+    mockAutoUpdater.checkForUpdates.mockRejectedValueOnce(new Error('network down'));
+    await updater.check();
+    expect(send).toHaveBeenCalledWith('updates:status', {
+      state: 'error',
+      message: 'network down',
+    });
+  });
+
+  test('check() respects isDev() and does not invoke autoUpdater', async () => {
+    const isDev = jest.fn(() => true);
+    const u2 = setupAutoUpdater({ send, isDev });
+    await u2.check();
+    expect(isDev).toHaveBeenCalled();
+    expect(mockAutoUpdater.checkForUpdates).not.toHaveBeenCalled();
+  });
+
+  test('install() calls quitAndInstall', () => {
+    updater.install();
+    expect(mockAutoUpdater.quitAndInstall).toHaveBeenCalledTimes(1);
+  });
+
+  test('forwarded events carry typed payloads', () => {
+    // Find each handler and invoke it; assert what got sent.
+    const map = new Map();
+    for (const [event, cb] of mockAutoUpdater.on.mock.calls) map.set(event, cb);
+
+    send.mockClear();
+    map.get('checking-for-update')();
+    expect(send).toHaveBeenLastCalledWith('updates:status', { state: 'checking' });
+
+    map.get('update-available')({ version: '4.13.0' });
+    expect(send).toHaveBeenLastCalledWith('updates:status', {
+      state: 'available',
+      version: '4.13.0',
+    });
+
+    map.get('download-progress')({ percent: 42.7 });
+    expect(send).toHaveBeenLastCalledWith('updates:status', {
+      state: 'downloading',
+      percent: 43,
+    });
+
+    map.get('update-downloaded')({ version: '4.13.0' });
+    expect(send).toHaveBeenLastCalledWith('updates:status', {
+      state: 'downloaded',
+      version: '4.13.0',
+    });
+
+    map.get('error')(new Error('boom'));
+    expect(send).toHaveBeenLastCalledWith('updates:status', {
+      state: 'error',
+      message: 'boom',
+    });
+  });
+});