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.

.agents/notes/implemented/testing/2026-07-24-web-gui-browser-e2e-lane.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write .agents/notes/implemented/testing/2026-07-24-web-gui-browser-e2e-lane.md
-2026-07-24-web-gui-browser-e2e-lane.md: 107dbddbfde8ad29e22d9cba04ce2b83c1d01383
-2026-07-24-web-gui-browser-e2e-lane.zh.md: e4132b2ebb3f30a9d540f47cf9416a13bc4aa9f3
+2026-07-24-web-gui-browser-e2e-lane.md: cdb7de52c50733d6650202ee2117916319940738
+2026-07-24-web-gui-browser-e2e-lane.zh.md: b850acf026502d054a9d8b2168f0b4f47f58f39b

.agents/notes/implemented/testing/2026-07-24-web-gui-browser-e2e-lane.md

@@ -28,7 +28,7 @@ The barrier stack for replay-mode browser assertions is, in order: (1) host-side
 
 No single-shot transient-DOM assertions: every hop from replay yield to React commit can coalesce chunks, so sampling `[data-streaming]` is a race by construction. Streaming incrementality is asserted from the persisted `assistant/chunk` events (model-visible ⟺ logged makes the log the authoritative proof). `dsh-llm-replay`'s opt-in `paceMs` (default absent = burst) is a realism knob so the browser observes genuinely incremental SSE; correctness never leans on it, and abort during a pace wait cancels promptly.
 
-Every scenario fails on any pageerror and on the client's connection-loss/gap-repair console warnings: the reconnect machine plus history resync would otherwise self-heal a dead SSE path and the suite would certify a broken wire. Scaffold `close()` calls the `ReplayHandle.assertConsumed()` teardown check (every recorded script bound, every cursor drained), converting silent underruns and shifted bindings into crisp diagnostics. No vitest retry on the lane; one chromium per file, fresh context per scenario, one host per scenario; viewport pinned; interaction selectors anchor on roles, `data-*` attributes, and visible text, while the frame and conversation-region captures use the existing CSS-module local-name anchors. Standard scenarios set `dsh.locale=en` before client boot so localized role locators and goldens use one explicit language; `settings-chrome.e2e.ts` alone leaves storage unset to cover the default Chinese state and both switch directions.
+Every scenario fails on any pageerror and on the client's connection-loss/gap-repair console warnings: the reconnect machine plus history resync would otherwise self-heal a dead SSE path and the suite would certify a broken wire. Scaffold `close()` calls the `ReplayHandle.assertConsumed()` teardown check (every recorded script bound, every cursor drained), converting silent underruns and shifted bindings into crisp diagnostics. No vitest retry on the lane; one chromium per file, fresh context per scenario, one host per scenario; viewport pinned; interaction selectors anchor on roles, `data-*` attributes, and visible text, while the frame and conversation-region captures use the existing CSS-module local-name anchors. Standard scenarios set `dsh.locale=en` before client boot so localized role locators and goldens use one explicit language; the scenarios asserting Chinese copy leave storage unset and open a `zh-CN` browser instead, because the client derives its initial locale from `navigator` ([browser-derived initial locale](../feature/2026-07-31-browser-derived-initial-locale.md)), and `settings-chrome.e2e.ts` additionally covers both switch directions and the English-browser default.
 
 ### Expected outputs
 

.agents/notes/implemented/testing/2026-07-24-web-gui-browser-e2e-lane.zh.md

@@ -28,7 +28,7 @@ Web GUI 以一条真实组装链交付——chromium 页面 → client 插件 bu
 
 不做单次瞬态 DOM 断言：从回放产出到 React 提交的每一跳都可能合并分片，采样 `[data-streaming]` 天然就是竞态。流式输出的增量性由持久化的 `assistant/chunk` 事件断言（模型可见 ⟺ 已记录，使日志成为权威证据）。`dsh-llm-replay` 的可选 `paceMs`（默认缺省 = 突发）只是让浏览器观察到真正增量 SSE 的真实感旋钮；正确性绝不依赖它，且节奏等待期间中止会即时取消。
 
-每个场景都会因任何 pageerror 或客户端的连接丢失/间隙修复控制台警告而失败：否则重连机制加历史重同步会把一条死掉的 SSE 通路自愈掉，套件反而认证了坏 wire。Scaffold 的 `close()` 调用 `ReplayHandle.assertConsumed()` 收尾检查（每个已录脚本都被绑定、每个游标都耗尽），把静默的少放与错绑变成清晰诊断。车道不设 vitest 重试；每文件一个 chromium、每场景一个新 context、每场景一个 host；视口固定；交互选择器锚定 role、`data-*` 属性和可见文本，而 frame 与会话区采集则使用既有的 CSS 模块局部类名锚点。常规场景在客户端启动前设置 `dsh.locale=en`，使本地化的 role 定位器和预期输出统一采用明确指定的语言；只有 `settings-chrome.e2e.ts` 不预设该存储项，以覆盖默认中文状态及双向切换。
+每个场景都会因任何 pageerror 或客户端的连接丢失/间隙修复控制台警告而失败：否则重连机制加历史重同步会把一条死掉的 SSE 通路自愈掉，套件反而认证了坏 wire。Scaffold 的 `close()` 调用 `ReplayHandle.assertConsumed()` 收尾检查（每个已录脚本都被绑定、每个游标都耗尽），把静默的少放与错绑变成清晰诊断。车道不设 vitest 重试；每文件一个 chromium、每场景一个新 context、每场景一个 host；视口固定；交互选择器锚定 role、`data-*` 属性和可见文本，而 frame 与会话区采集则使用既有的 CSS 模块局部类名锚点。常规场景在客户端启动前设置 `dsh.locale=en`，使本地化的 role 定位器和预期输出统一采用明确指定的语言；断言中文文案的场景则不预设该存储项，改为开启 `zh-CN` 浏览器，因为客户端的初始 locale 由 `navigator` 推导（[由浏览器推导初始 locale](../feature/2026-07-31-browser-derived-initial-locale.md)），而 `settings-chrome.e2e.ts` 还额外覆盖双向切换与英文浏览器默认态。
 
 ### 预期输出