feat(locale): derive the initial Settings language from the browser

A first visit resolved to Chinese regardless of the browser: LocaleService
read `dsh.locale` and fell straight back to `zh` when nothing was stored,
ignoring the languages the browser already states it reads.

The initial locale now resolves through three ordered sources — the persisted
preference, then `navigator` (first entry of the ordered language list whose
primary subtag names a shipped locale, so `zh-Hans-CN` -> zh and `en-GB` ->
en), then `FALLBACK_LOCALE`. An explicit choice still wins and nothing writes
the detected locale back to storage, so "has the user chosen?" stays a
question only the stored value answers.

Specs asserting the shipped Chinese copy now state the browser they assume:
the web e2e scenarios open their page with `locale: ZH_BROWSER_LOCALE`, and
package specs pin it through the new `pinBrowserLanguages` test helper.
`settings-chrome.e2e.ts` gains an English-browser scenario as the
assembled-app proof.

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

@@ -8,12 +8,18 @@
  * holes (sidebar.workspaces / sidebar.settings) have no registrant here, so
  * the snapshots pin the shell chrome itself.
  */
-import { afterEach, describe, expect, it, vi } from 'vitest'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import { act, cleanup, waitFor } from '@testing-library/react'
-import { SlotTestRuntime } from '@deepseek-ai/dsh-client-test-runtime'
+import { SlotTestRuntime, pinBrowserLanguages } 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() })
+
 afterEach(cleanup)
 
 /**