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。

tests/locales.spec.ts

@@ -6,7 +6,7 @@
  * interpolation.
  */
 import { afterEach, describe, expect, it } from 'vitest'
-import { LOCALE_NS, attachLocale, en, isZh, relativeTime, t, zh } from '../src/client/locales.ts'
+import { LOCALE_NS, attachBetterLocale, attachLocale, en, isZh, ja, relativeTime, t, zh } from '../src/client/locales.ts'
 
 /** Minimal structural fake of the DSH LocaleService face the sidebar uses. */
 class FakeLocale {
@@ -35,6 +35,7 @@ function stubNavigatorLanguage(lang: string | undefined): void {
 
 afterEach(() => {
   attachLocale(undefined)
+  attachBetterLocale(undefined)
   stubNavigatorLanguage(undefined)
 })
 
@@ -106,4 +107,121 @@ describe('locales (DSH i18n following)', () => {
   it('keeps the zh and en dictionaries key-set-equal', () => {
     expect(Object.keys(en).sort()).toEqual(Object.keys(zh).sort())
   })
+
+  it('keeps the ja dictionary key-set-equal to zh', () => {
+    expect(Object.keys(ja).sort()).toEqual(Object.keys(zh).sort())
+  })
+})
+
+/** Minimal fake of the better-locale override store (the subset t() reads). */
+class FakeBetterLocale {
+  active: string | undefined = undefined
+  private readonly dict: Record<string, Record<string, Record<string, string>>>
+  constructor(dict: Record<string, Record<string, Record<string, string>>>) {
+    this.dict = dict
+  }
+  getOverride(_dshActive: string, ns: string, key: string): string | undefined {
+    if (this.active === undefined) return undefined
+    return this.dict[ns]?.[this.active]?.[key]
+  }
+  subscribe(_listener: () => void): () => void {
+    return () => {}
+  }
+}
+
+describe('locales (better-locale override)', () => {
+  it('returns the override text when an override is active and the key exists', () => {
+    const locale = new FakeLocale()
+    locale.switchTo('en')
+    attachLocale(locale)
+
+    const betterLocale = new FakeBetterLocale({
+      [LOCALE_NS]: { ja: { explorer: 'エクスプローラー' } },
+    })
+    betterLocale.active = 'ja'
+    attachBetterLocale(betterLocale)
+
+    expect(t('explorer')).toBe('エクスプローラー')
+    expect(isZh()).toBe(false)
+  })
+
+  it('falls back to the zh/en chain when override is active but the key is missing', () => {
+    const locale = new FakeLocale()
+    locale.switchTo('en')
+    attachLocale(locale)
+
+    const betterLocale = new FakeBetterLocale({
+      [LOCALE_NS]: { ja: { explorer: 'エクスプローラー' } },
+    })
+    betterLocale.active = 'ja'
+    attachBetterLocale(betterLocale)
+
+    // 'git' has no ja entry in this fake; should fall back to the en text.
+    expect(t('git')).toBe('Source Control')
+  })
+
+  it('falls back to the zh/en chain when no override is active', () => {
+    const locale = new FakeLocale()
+    locale.switchTo('zh')
+    attachLocale(locale)
+
+    const betterLocale = new FakeBetterLocale({
+      [LOCALE_NS]: { ja: { explorer: 'エクスプローラー' } },
+    })
+    // active is undefined — no override.
+    attachBetterLocale(betterLocale)
+
+    expect(t('explorer')).toBe('资源管理器')
+    expect(isZh()).toBe(true)
+  })
+
+  it('returns to the zh/en chain after detaching the override store', () => {
+    const locale = new FakeLocale()
+    locale.switchTo('en')
+    attachLocale(locale)
+
+    const betterLocale = new FakeBetterLocale({
+      [LOCALE_NS]: { ja: { explorer: 'エクスプローラー' } },
+    })
+    betterLocale.active = 'ja'
+    attachBetterLocale(betterLocale)
+    expect(t('explorer')).toBe('エクスプローラー')
+
+    attachBetterLocale(undefined)
+    expect(t('explorer')).toBe('Explorer')
+    expect(isZh()).toBe(false)
+  })
+
+  it('interpolates {name} placeholders in the override text', () => {
+    const locale = new FakeLocale()
+    locale.switchTo('en')
+    attachLocale(locale)
+
+    const betterLocale = new FakeBetterLocale({
+      [LOCALE_NS]: { ja: { timeMinutesAgo: '{n} 分前' } },
+    })
+    betterLocale.active = 'ja'
+    attachBetterLocale(betterLocale)
+
+    expect(t('timeMinutesAgo', { n: 5 })).toBe('5 分前')
+  })
+
+  it('ships a ja dictionary whose every key resolves to a non-empty string', () => {
+    // Smoke-check the shipped ja dict: every key in zh must have a non-empty
+    // ja entry. This catches copy-paste mistakes where a key was added to zh
+    // but its ja translation was left blank or missing.
+    const locale = new FakeLocale()
+    locale.switchTo('en')
+    attachLocale(locale)
+
+    const betterLocale = new FakeBetterLocale({ [LOCALE_NS]: { ja } })
+    betterLocale.active = 'ja'
+    attachBetterLocale(betterLocale)
+
+    for (const key of Object.keys(zh) as (keyof typeof zh)[]) {
+      const text = t(key)
+      expect(text, `ja translation for "${key}"`).toBeTruthy()
+      expect(text, `ja translation for "${key}"`).not.toBe(key)
+    }
+  })
 })