i18n: fix hardcoded French in Statistics and Shortcuts panel

- dashboard.js: Fichiers/Tags uniques/Taille totale/Vaults -> t() calls
- config.js: Tags uniques/Vaults in About -> t() calls
- index.html: Raccourcis & Astuces -> data-i18n
- Added 5 new keys: dashboard.stats_*, help.shortcuts_title

frontend/locales/fr.json

@@ -1035,5 +1035,10 @@
   "config.force_reindex": "Forcer réindexation",
   "search.previous": "← Précédent",
   "viewer.copied": "Copié !",
-  "viewer.shared_doc_title": "Document partagé — cliquer pour gérer"
+  "viewer.shared_doc_title": "Document partagé — cliquer pour gérer",
+  "dashboard.stats_files": "Fichiers",
+  "dashboard.stats_size": "Taille totale",
+  "dashboard.stats_tags": "Tags uniques",
+  "dashboard.stats_vaults": "Vaults",
+  "help.shortcuts_title": "Raccourcis & Astuces"
 }