fix(i18n): use translation keys for upload notification titles

Replace hardcoded English/Spanish strings with proper i18n translations
for upload notification titles and file count labels.

- Add upload.files translation key to all locale files
- Update notifications.js to use window.i18n.t() for:
  - upload.uploading title
  - upload.files file count label
  - upload.complete completion message

Fixes #108

static/locales/ko.json

@@ -27,6 +27,7 @@
     "upload_folder": "폴더 업로드",
     "upload.uploading": "업로드 중...",
     "upload.complete": "{count} / {total} 업로드 완료",
+    "upload.files": "파일",
     "rename": "이름 변경",
     "move": "이동...",
     "move_to": "이동 대상",