Add Japanese locale (ja) support (#1810)

* Add Japanese locale (ja) support

- New: ja.ts with 2624 UI translation keys
- New: ja.settings.ts with 1789 settings translation keys
- Registered ja in runtime.ts (Locale type, LOCALES, LOCALE_LABEL_KEYS, normalizeLocale)
- Added ja-JP BCP-47 mapping in intl.ts
- Added JA_MESSAGES in bootstrap.ts
- Added ja to lazy-load dictionary map in store.ts
- Updated messages.test.ts with ja dictionary
- Added common.language.japanese to all 8 existing locale files

* fix(i18n): complete Japanese locale updates

* test(i18n): enforce locale key parity

* docs: add Japanese documentation

---------

Co-authored-by: yuchi0531 <yuchi0531@users.noreply.github.com>
Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/lib/i18n/bootstrap.ts

@@ -57,7 +57,30 @@ const FR_MESSAGES: BootstrapMessages = {
   loadingData: (providersText, agentsText) => `Chargement des données (${providersText}, ${agentsText})…`,
 };
 
-export const getBootstrapMessages = (locale: Locale): BootstrapMessages => (locale === 'fr' ? FR_MESSAGES : EN_MESSAGES);
+const JA_MESSAGES: BootstrapMessages = {
+  startingApi: 'OpenCode API を起動中…',
+  initializing: '初期化中…',
+  connecting: '接続中…',
+  connected: '接続完了！',
+  connectionError: '接続エラー',
+  disconnected: '切断されました',
+  reconnecting: '再接続中…',
+  initialDataLoadFailed: 'OpenCode に接続しましたが、初期データの読み込みに失敗しました。',
+  cliNotFound: 'OpenCode CLI が見つかりません。先にインストールしてください。',
+  providersReady: '✓ プロバイダ',
+  providersLoading: '… プロバイダ',
+  agentsReady: '✓ エージェント',
+  agentsLoading: '… エージェント',
+  startingDevServer: (hostLabel) => `Webview 開発サーバーを起動中 (${hostLabel})...`,
+  waitingDevServer: (hostLabel, attempt) => `Webview 開発サーバーを待機中 (${hostLabel})... 試行 ${attempt}`,
+  loadingData: (providersText, agentsText) => `データを読み込み中 (${providersText}, ${agentsText})…`,
+};
+
+export const getBootstrapMessages = (locale: Locale): BootstrapMessages => {
+  if (locale === 'fr') return FR_MESSAGES;
+  if (locale === 'ja') return JA_MESSAGES;
+  return EN_MESSAGES;
+};
 
 export const readStoredLocaleForBootstrap = (): Locale => {
   if (typeof window === 'undefined') {