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.

apps/web/tests/support.ts

@@ -9,11 +9,18 @@ export const DIST_INDEX = fileURLToPath(new URL('../dist/index.html', import.met
 
 export const REPO_ROOT = fileURLToPath(new URL('../../..', import.meta.url))
 
+/**
+ * Browser language a page must advertise to boot into the product's Chinese
+ * surface: with no stored preference the client derives its initial locale
+ * from the browser, and Playwright's default browser asks for English.
+ */
+export const ZH_BROWSER_LOCALE = 'zh-CN'
+
 /**
  * Open the standard browser-test page with English selected before client
  * boot. This keeps role locators and goldens deterministic across localized
- * component migrations; the settings locale scenario deliberately bypasses
- * this helper to cover the product's default Chinese state.
+ * component migrations; the scenarios asserting the Chinese surface bypass
+ * this helper and advertise {@link ZH_BROWSER_LOCALE} instead.
  * @param browser - Playwright browser owning the page.
  * @param height - Viewport height; width is fixed to the lane baseline.
  * @returns the initialized page.