refactor(desktop): share shell settings path resolution

apps/desktop/src/settings.ts

@@ -3,7 +3,7 @@
  * Ownership: .agents/notes/implemented/architecture/2026-10-08-desktop-shell-configuration.md.
  */
 import { mkdirSync, readFileSync, writeFileSync } from 'node:fs'
-import { join } from 'node:path'
+import { dirname, join } from 'node:path'
 
 /** Resolved shell preferences; absent fields use the defaults without rewriting existing files. */
 export interface DesktopSettings {
@@ -16,20 +16,28 @@ function record(value: unknown): value is Record<string, unknown> {
   return typeof value === 'object' && value !== null && !Array.isArray(value)
 }
 
+/**
+ * Resolve the shared document used by shell preferences and update channels.
+ * @param userData - Electron's userData directory, independent of DSH_HOME.
+ * @returns Absolute settings path when userData is absolute; no filesystem changes.
+ */
+export function desktopSettingsPath(userData: string): string {
+  return join(userData, 'desktop', 'settings.json')
+}
+
 /**
  * Read shell settings, creating the default document exclusively when missing.
  * @param userData - Electron's userData directory, independent of DSH_HOME.
  * @returns Validated settings, or defaults when reading, creating or validating fails; changes require a restart.
  */
 export function readDesktopSettings(userData: string): DesktopSettings {
-  const directory = join(userData, 'desktop')
-  const path = join(directory, 'settings.json')
+  const path = desktopSettingsPath(userData)
   const defaults: DesktopSettings = { updates: { allowTestAuthPopupWindow: false } }
   try {
     let raw: string
     try { raw = readFileSync(path, 'utf8') } catch (error) {
       if ((error as NodeJS.ErrnoException).code !== 'ENOENT') throw error
-      mkdirSync(directory, { recursive: true, mode: 0o700 })
+      mkdirSync(dirname(path), { recursive: true, mode: 0o700 })
       try { writeFileSync(path, `${JSON.stringify(defaults, null, 2)}\n`, { flag: 'wx', mode: 0o600 }) } catch (error) {
         // An existing document belongs to the user, including one created during initialization.
         if ((error as NodeJS.ErrnoException).code !== 'EEXIST') throw error

apps/desktop/src/update-settings.ts

@@ -1,7 +1,8 @@
 /** Startup-only file reads and explicit UI writes for shell-owned update channels. */
 import { readFile, mkdir } from 'node:fs/promises'
-import { dirname, join } from 'node:path'
+import { dirname } from 'node:path'
 import { withFileLock, writeFileAtomic } from '@deepseek-ai/dsh-atomic-write'
+import { desktopSettingsPath } from './settings.ts'
 import type { DesktopUpdateChannel } from './update-channel.ts'
 
 /** A settings operation failed without applying a new runtime channel. */
@@ -43,7 +44,7 @@ export class DesktopUpdateSettings {
    * @param packagedChannel - Default selected by the installer build input.
    */
   constructor(userData: string, private readonly packagedChannel: DesktopUpdateChannel) {
-    this.path = join(userData, 'desktop', 'settings.json')
+    this.path = desktopSettingsPath(userData)
   }
 
   /** @returns The startup channel, or a failure that must suspend ordinary updates. */

apps/desktop/tests/update-settings.spec.ts

@@ -2,6 +2,7 @@ import { mkdtemp, mkdir, readFile, writeFile, rm } from 'node:fs/promises'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
 import { expect, it, onTestFinished } from 'vitest'
+import { readDesktopSettings } from '../src/settings.ts'
 import { DesktopUpdateSettings, DesktopUpdateSettingsError } from '../src/update-settings.ts'
 
 async function fixture() {
@@ -48,3 +49,16 @@ it('reports unreadable configuration instead of silently choosing a default', as
   await mkdir(settings.path)
   expect(await settings.load()).toMatchObject({ kind: 'error', error: { kind: 'read' } })
 })
+
+it('updates the document initialized by shell settings without losing the popup preference', async () => {
+  const root = await mkdtemp(join(tmpdir(), 'desktop-shared-settings-'))
+  onTestFinished(() => rm(root, { recursive: true, force: true }))
+  expect(readDesktopSettings(root).updates.allowTestAuthPopupWindow).toBe(false)
+  const settings = new DesktopUpdateSettings(root, 'latest')
+  await settings.save('nightly')
+  expect(JSON.parse(await readFile(settings.path, 'utf8'))).toEqual({
+    updates: { allowTestAuthPopupWindow: false, channel: 'nightly' },
+  })
+  expect(readDesktopSettings(root).updates.allowTestAuthPopupWindow).toBe(false)
+  expect(await settings.load()).toEqual({ kind: 'ready', channel: 'nightly' })
+})