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/lib/schemas/settings.ts

@@ -13,7 +13,7 @@ export const settingsSchema = z.object({
   minimizeToTrayOnClose: z.boolean(),
   enableClaudePluginIntegration: z.boolean().optional(),
   launchOnStartup: z.boolean().optional(),
-  language: z.enum(["en", "zh"]).optional(),
+  language: z.enum(["en", "zh", "ja"]).optional(),
 
   // 设备级目录覆盖
   claudeConfigDir: directorySchema.nullable().optional(),