fix(locale): gate browser detection on window and tolerate a missing languages list

Node >= 21 exposes a global `navigator` reporting the machine's own language,
so gating detection on `navigator` let a non-browser boot of the client tree
resolve to `en` instead of the documented fallback; `window` is the browser
test. `navigator.languages` is spec-required but absent on some embedders and
older WebViews, where spreading it would throw at boot, so the walk tolerates
its absence and `navigator.language` covers that host.

The per-spec pin boilerplate collapses into one suite-level
`usePinnedBrowserLanguages('zh-CN')`, which owns the rationale in
dsh-client-test-runtime, and the English-browser e2e scenario now clears the
console warnings channel too — its page has no closing inventory spec.

packages/client/ui-sidebar/tests/sidebar-snapshot.spec.tsx

@@ -8,17 +8,15 @@
  * holes (sidebar.workspaces / sidebar.settings) have no registrant here, so
  * the snapshots pin the shell chrome itself.
  */
-import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 import { act, cleanup, waitFor } from '@testing-library/react'
-import { SlotTestRuntime, pinBrowserLanguages } from '@deepseek-ai/dsh-client-test-runtime'
+import { SlotTestRuntime, usePinnedBrowserLanguages } from '@deepseek-ai/dsh-client-test-runtime'
 import { LocaleService } from '@deepseek-ai/dsh-client-locale/client'
 import { apply, inject } from '@deepseek-ai/dsh-client-ui-sidebar/client'
 
 // The service reads its initial locale from the browser; these specs assert
 // the shipped Chinese copy, so they state the browser they assume.
-let restoreLanguages: () => void
-beforeEach(() => { restoreLanguages = pinBrowserLanguages('zh-CN') })
-afterEach(() => { restoreLanguages() })
+usePinnedBrowserLanguages('zh-CN')
 
 afterEach(cleanup)