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-models/tests/apply.spec.ts

@@ -1,19 +1,17 @@
 /** Models section registration: declaration-aware deferral, the locale-following label thunk, and HMR recovery. */
 import { Context } from 'cordis'
-import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { describe, expect, it, vi } from 'vitest'
 import { resolveSlotLabel } from '@deepseek-ai/dsh-client-ui-slots'
 import { SlotsService } from '@deepseek-ai/dsh-client-runtime/client'
 import { LocaleService } from '@deepseek-ai/dsh-client-locale/client'
-import { pinBrowserLanguages } from '@deepseek-ai/dsh-client-test-runtime'
+import { usePinnedBrowserLanguages } from '@deepseek-ai/dsh-client-test-runtime'
 import { apply, inject, refreshIfLoaded } from '@deepseek-ai/dsh-client-ui-models/client'
 import { ModelsSection } from '../src/client/ModelsSection.tsx'
 import { DeepSeekOnboardingDialog } from '../src/client/DeepSeekOnboardingDialog.tsx'
 
 // 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')
 
 async function bench() {
   const ctx = new Context()