chore: Translate en.json in fr [Manual Sync]

99% of minimum 50% translated source file: 'en.json'
on 'fr'.

Sync of partially translated files: 
untranslated content is included with an empty translation 
or source language content depending on file format

frontend/src/i18n/fr.json

@@ -223,7 +223,7 @@
     "shareDeleted": "Partage supprimé !",
     "singleClick": "Utiliser un simple clic pour ouvrir les fichiers et les dossiers",
     "themes": {
-      "default": "",
+      "default": "System default",
       "dark": "Sombre",
       "light": "Clair",
       "title": "Thème"