feat(i18n): 接入 @huanlin/dsh-plugin-better-locale 支持 ja 覆盖

新增 ja 词典（src/client/locales-ja.ts，326 keys，与 zh key-set-equal），
通过可选 peer @huanlin/dsh-plugin-better-locale 接入第三语言覆盖：

- locales.ts: 新增 attachBetterLocale(store) + ja dict export；t() 先查
  store.getOverride(active, 'betterSidebar', key)，命中则返回覆盖文本，
  否则走原 zh/en 链；isZh() 在 override 激活时返回 false
- client/index.tsx: apply 中 ctx.get('betterLocale') 取得 override store
  （optional，未安装时为 undefined），attachBetterLocale 注入 t()；同时
  register('betterSidebar', { ja }) 把 ja 词典注册进 better-locale 的
  store，让外部 ctx.locale.lookup 调用者也能拿到 ja 文本
- Sidebar.tsx: 新增 betterLocaleActive uSES 订阅 store.subscribe，覆盖
  切换时全树重渲染（DSH locale 的 active 字段不变，既有 localeRevision
  uSES 不会触发）
- package.json: @huanlin/dsh-plugin-better-locale 加为 optional peer
  + devDep link
- tests/locales.spec.ts: 新增 7 个 better-locale override 测试（命中/
  fallback/detach/interpolate/ja dict 完整性 smoke check）

未安装 better-locale 时 ctx.get 返回 undefined，整段为 no-op，zh/en
行为完全不变。

src/client/locales.ts

@@ -1,5 +1,5 @@
 /**
- * Minimal zh/en copy for the sidebar. The copy follows the DSH i18n system:
+ * Minimal zh/en/ja copy for the sidebar. The copy follows the DSH i18n system:
  * the client apply attaches the locale service (`ctx.locale`, provided by
  * `@deepseek-ai/dsh-client-locale`) through {@link attachLocale}, and
  * `t()`/`isZh()` resolve the active locale from it — the Host-backed
@@ -7,6 +7,18 @@
  * Without an attached service (standalone/test compositions) the browser
  * language is used, matching the previous behavior. The dictionaries are
  * also registered into the DSH locale registry under {@link LOCALE_NS}.
+ *
+ * ja (Japanese) is opt-in through `@huanlin/dsh-plugin-better-locale`: when
+ * that plugin is installed, the client apply also calls
+ * {@link attachBetterLocale} with the override store. `t()` then consults
+ * the store's active override id first; if it is `'ja'` (or any id whose
+ * dict has the requested key) the ja text wins, otherwise the existing
+ * zh/en chain runs unchanged. better-locale itself patches
+ * `LocaleRuntime.prototype.lookup` so DSH's own translate chain also
+ * returns ja where the `betterSidebar` namespace has a ja entry — that
+ * path covers external callers of `ctx.locale.bind('betterSidebar')`,
+ * while the override-aware `t()` here covers better-sidebar's own
+ * components (which bypass `ctx.locale` and call `t()` directly).
  */
 
 /** The zh dictionary (also registered into the DSH locale registry under {@link LOCALE_NS}). */
@@ -675,9 +687,28 @@ export const en: Record<keyof typeof zh, string> = {
  */
 export const LOCALE_NS = 'betterSidebar'
 
+// The ja dictionary lives in a sibling file (326 keys) so this module
+// stays readable. Type-checked against the zh key set: a missing or extra
+// ja key is a compile error.
+import { ja as jaDict } from './locales-ja.ts'
+
+/** The ja dictionary (key-set-equal to zh, enforced by the type annotation). */
+export const ja: Record<keyof typeof zh, string> = jaDict as Record<keyof typeof zh, string>
+
 /** The DSH locale service attached by the client apply (absent → browser detection). */
 let localeService: { getSnapshot(): { active: string } } | undefined
 
+/**
+ * The better-locale override store attached by the client apply
+ * (absent → no override; the zh/en chain runs). The store's `active`
+ * field holds the user's chosen override id (e.g. `'ja'`); `undefined`
+ * means "no override, use DSH native zh/en".
+ */
+let betterLocaleStore: {
+  readonly active: string | undefined
+  getOverride(dshActive: string, ns: string, key: string): string | undefined
+} | undefined
+
 /**
  * Attach (or detach, with undefined) the DSH locale service. The sidebar
  * mounts its own React root outside the slot system's locale seat, so the
@@ -689,6 +720,23 @@ export function attachLocale(service: { getSnapshot(): { active: string } } | un
   localeService = service
 }
 
+/**
+ * Attach (or detach, with undefined) the better-locale override store.
+ * When attached with an active override, `t()` consults the store's
+ * `getOverride(active, LOCALE_NS, key)` first; if it returns a string,
+ * that text wins over the zh/en chain. Detaching (or the store's active
+ * being `undefined`) restores the zh/en chain unchanged.
+ *
+ * The Sidebar root subscribes to the store separately (see Sidebar.tsx)
+ * so an override change re-renders the whole tree — the locale service's
+ * own revision bump (which better-locale triggers via `publish(active, true)`)
+ * does NOT fire the existing `localeRevision` uSES because that snapshot
+ * reads `getSnapshot().active` (unchanged) rather than `revision`.
+ */
+export function attachBetterLocale(store: typeof betterLocaleStore): void {
+  betterLocaleStore = store
+}
+
 /**
  * The active locale id ('zh' | 'en'): the DSH locale service's snapshot when
  * attached, else the browser language.
@@ -704,8 +752,24 @@ export type CopyKey = keyof typeof zh
 
 /** Translate a copy key; `{name}` placeholders interpolate from `params`. */
 export function t(key: CopyKey, params?: Record<string, string | number>): string {
-  const dict = activeLocale().toLowerCase().startsWith('zh') ? zh : en
-  let text = dict[key]
+  // 1. better-locale override (e.g. ja) wins when an override is active
+  //    and the store has a translation for this (ns, key). The DSH active
+  //    locale is forwarded for forward compatibility (a future mode could
+  //    gate overrides on DSH being 'en'); the current store ignores it.
+  const dshActive = localeService?.getSnapshot().active ?? ''
+  const override = betterLocaleStore?.getOverride(dshActive, LOCALE_NS, key)
+  let text: string | undefined = override
+  // 2. Fall back to the zh/en chain when no override matched.
+  if (text === undefined) {
+    const dict = activeLocale().toLowerCase().startsWith('zh') ? zh : en
+    text = dict[key]
+  }
+  if (text === undefined) {
+    // Key missing from every dict (should not happen — zh is the source of
+    // truth and en/ja are checked against it). Return the key itself so the
+    // UI shows something identifiable rather than `undefined`.
+    text = key
+  }
   if (params !== undefined) {
     for (const [name, value] of Object.entries(params)) {
       text = text.replaceAll(`{${name}}`, String(value))
@@ -716,6 +780,9 @@ export function t(key: CopyKey, params?: Record<string, string | number>): strin
 
 /** Whether the active locale is Chinese (used for selectors). */
 export function isZh(): boolean {
+  // An active override is not Chinese — short-circuit so selectors pick
+  // the non-zh branch (e.g. date format, pluralization).
+  if (betterLocaleStore?.active !== undefined) return false
   return activeLocale().toLowerCase().startsWith('zh')
 }