fix(desktop): switching back to the stable update channel no longer errors (#197)

## Problem
On the Updates settings page, choosing **Stable** after Beta or Nightly
shows `Channel must be a string, but got: null`.
`configureExplicitConsent` assigned `null` to `autoUpdater.channel`;
electron-updater's setter rejects non-strings once a channel has been
set.

## What changed
- `apps/desktop/src/updater.ts`: stable now sets electron-updater's
default channel name `latest` explicitly (same `latest-mac.yml`, no
behavior change for fresh installs).
- `apps/desktop/tests/updater.spec.ts`: the `electron-updater` mock
enforces the real `channel` setter rules, so the existing channel-switch
test fails on the old code (verified) and asserts `latest` now.

[skip changeset] — desktop app, not a changesets-managed package.

Author: elkaix

apps/desktop/src/updater.ts

@@ -8,6 +8,12 @@ import { gt, valid } from 'semver'
 const { autoUpdater } = electronUpdater
 const UPDATE_SETTINGS_FILE = 'update-settings.json'
 const UPDATES_UNAVAILABLE_MESSAGE = 'Updates are not available for this build'
+/**
+ * electron-updater's default channel. Its `channel` setter rejects `null` once a
+ * channel has been set, so switching back to stable must name the channel
+ * explicitly instead of clearing it.
+ */
+const STABLE_UPDATER_CHANNEL = 'latest'
 const INITIAL_CHECK_DELAY_MS = 10_000
 const CHECK_INTERVAL_MS = 4 * 60 * 60 * 1_000
 const RELEASE_REPOSITORY_PATH = '/PyModel/pythinker-desktop-releases/releases/tag/'
@@ -258,7 +264,7 @@ function hasUpdateConfig(): boolean {
 function configureExplicitConsent(): void {
   autoUpdater.autoDownload = false
   autoUpdater.autoInstallOnAppQuit = false
-  autoUpdater.channel = settings.channel === 'stable' ? null : settings.channel
+  autoUpdater.channel = settings.channel === 'stable' ? STABLE_UPDATER_CHANNEL : settings.channel
   autoUpdater.allowPrerelease = settings.channel !== 'stable'
   autoUpdater.allowDowngrade = false
 }

apps/desktop/tests/updater.spec.ts

@@ -15,6 +15,17 @@ vi.mock('electron', () => ({
 vi.mock('electron-updater', () => ({
   default: {
     autoUpdater: {
+      _channel: null as string | null,
+      get channel(): string | null {
+        return this._channel
+      },
+      set channel(value: string | null) {
+        if (this._channel !== null) {
+          if (typeof value !== 'string') throw new Error(`Channel must be a string, but got: ${String(value)}`)
+          if (value.length === 0) throw new Error('Channel must be not an empty string')
+        }
+        this._channel = value
+      },
       on: vi.fn(),
       checkForUpdates: vi.fn(),
       downloadUpdate: vi.fn(() => Promise.resolve([])),
@@ -58,7 +69,7 @@ afterEach(() => {
   autoUpdater.autoInstallOnAppQuit = undefined as unknown as boolean
   autoUpdater.allowPrerelease = undefined as unknown as boolean
   autoUpdater.allowDowngrade = undefined as unknown as boolean
-  autoUpdater.channel = null
+  ;(autoUpdater as unknown as { _channel: string | null })._channel = null
 })
 
 function temporaryDirectory(): string {
@@ -225,7 +236,7 @@ describe('strict update consent', () => {
     expect(setLocalUpdateChannel('nightly')).toMatchObject({ channel: 'nightly', status: 'idle' })
     expect(setLocalNotifyUpdate(false)).toMatchObject({ notifyUpdate: false })
     expect(setLocalUpdateChannel('stable')).toMatchObject({ channel: 'stable', status: 'idle' })
-    expect(localAutoUpdater.channel).toBeNull()
+    expect(localAutoUpdater.channel).toBe('latest')
     expect(localAutoUpdater.allowPrerelease).toBe(false)
     expect(localAutoUpdater.allowDowngrade).toBe(false)
     expect(localAutoUpdater.autoDownload).toBe(false)