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/Sidebar.tsx

@@ -188,6 +188,37 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
   )
   void localeRevision
 
+  // better-locale override freshness: when @huanlin/dsh-plugin-better-locale
+  // is installed and the user picks an override language (e.g. ja), the
+  // store's `active` changes but the DSH locale's `active` does NOT —
+  // better-locale keeps the dsh active value (zh/en) unchanged and only
+  // patches `LocaleRuntime.prototype.lookup`. The localeRevision uSES
+  // above reads `getSnapshot().active`, so it sees no change and skips
+  // re-render. This second uSES reads the better-locale store's `active`
+  // directly, so an override switch fires a full re-render and t() picks
+  // up the new override text. Optional: ctx.get returns undefined when
+  // better-locale is absent, in which case this is a no-op uSES.
+  type BetterLocaleStore = {
+    readonly active: string | undefined
+    subscribe(listener: () => void): () => void
+  }
+  const betterLocaleStore = (ctx as unknown as {
+    get(name: 'betterLocale'): BetterLocaleStore | undefined
+  }).get('betterLocale')
+  const betterLocaleActive = useSyncExternalStore(
+    useMemo(() => {
+      const store = betterLocaleStore
+      if (store === undefined) return (_cb: () => void) => () => {}
+      return (callback: () => void) => store.subscribe(callback)
+    }, [betterLocaleStore]),
+    useMemo(() => {
+      const store = betterLocaleStore
+      if (store === undefined) return () => undefined
+      return () => store.active
+    }, [betterLocaleStore]),
+  )
+  void betterLocaleActive
+
   // Tab-registry revision: TabContent memo cells must pick up a descriptor
   // a plugin registers/disposes after mount (the + menu / icons already read
   // the registry at render). Rare events (plugin (un)mount), so one full