feat(i18n): add Japanese language support

- Add complete Japanese translation file (ja.json)
- Update frontend types and hooks to support "ja" language option
- Add Japanese tray menu texts in Rust backend
- Add Japanese option to language settings component
- Update Zod schema to include Japanese language validation
- Add test case for Japanese language preference
- Update i18n documentation to reflect three-language support

src/types.ts

@@ -109,7 +109,7 @@ export interface Settings {
   // 是否开机自启
   launchOnStartup?: boolean;
   // 首选语言（可选，默认中文）
-  language?: "en" | "zh";
+  language?: "en" | "zh" | "ja";
 
   // ===== 设备级目录覆盖 =====
   // 覆盖 Claude Code 配置目录（可选）