localization: Update Japanese and Korean translations; add missing Korean locale files

- Updated Japanese translations in config.json, core.json, prompts.json, and startup.json for improved clarity and accuracy.
- Added new Korean locale files for config.json, core.json, prompts.json, and startup.json to support Korean language users.
- Enhanced Chinese translations in core.json and startup.json to include missing resource not found messages and WebUI related texts.

locales/ja-JP/core.json

@@ -1,7 +1,8 @@
 {
-  "core.operation_failed": "操作失败: {error}",
+  "core.not_found": "リソースが見つかりません",
+  "core.operation_failed": "操作に失敗しました: {error}",
   "core.tasks_cancelled": {
-    "one": "已取消 {count} 个任务",
-    "other": "已取消 {count} 个任务"
+    "one": "{count} 件のタスクをキャンセルしました",
+    "other": "{count} 件のタスクをキャンセルしました"
   }
 }