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/nl.json

@@ -27,6 +27,7 @@
     "upload_folder": "Map uploaden",
     "upload.uploading": "Uploaden...",
     "upload.complete": "{count} / {total} geüpload",
+    "upload.files": "bestanden",
     "rename": "Hernoemen",
     "move": "Verplaatsen naar...",
     "move_to": "Verplaatsen naar",