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.

pnpm-lock.yaml

@@ -1218,6 +1218,9 @@ importers:
       '@deepseek-ai/dsh-client-runtime':
         specifier: workspace:^
         version: link:../runtime
+      '@deepseek-ai/dsh-client-test-runtime':
+        specifier: workspace:^
+        version: link:../test-runtime
       '@deepseek-ai/dsh-client-ui-layout':
         specifier: workspace:^
         version: link:../ui-layout
@@ -1386,6 +1389,9 @@ importers:
       '@deepseek-ai/dsh-client-schema-form':
         specifier: workspace:^
         version: link:../schema-form
+      '@deepseek-ai/dsh-client-test-runtime':
+        specifier: workspace:^
+        version: link:../test-runtime
       '@deepseek-ai/dsh-client-ui-primitives':
         specifier: workspace:^
         version: link:../ui-primitives
@@ -1641,6 +1647,9 @@ importers:
       '@deepseek-ai/dsh-client-runtime':
         specifier: workspace:^
         version: link:../runtime
+      '@deepseek-ai/dsh-client-test-runtime':
+        specifier: workspace:^
+        version: link:../test-runtime
       '@deepseek-ai/dsh-client-ui-primitives':
         specifier: workspace:^
         version: link:../ui-primitives
@@ -1797,6 +1806,9 @@ importers:
       '@deepseek-ai/dsh-client-runtime':
         specifier: workspace:^
         version: link:../runtime
+      '@deepseek-ai/dsh-client-test-runtime':
+        specifier: workspace:^
+        version: link:../test-runtime
       '@deepseek-ai/dsh-client-ui-primitives':
         specifier: workspace:^
         version: link:../ui-primitives
@@ -3308,6 +3320,9 @@ importers:
       '@deepseek-ai/dsh-client-runtime':
         specifier: workspace:^
         version: link:../../client/runtime
+      '@deepseek-ai/dsh-client-test-runtime':
+        specifier: workspace:^
+        version: link:../../client/test-runtime
       '@deepseek-ai/dsh-client-ui-primitives':
         specifier: workspace:^
         version: link:../../client/ui-primitives