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/en.json

@@ -1035,5 +1035,10 @@
   "config.force_reindex": "Force reindex",
   "search.previous": "← Previous",
   "viewer.copied": "Copied!",
-  "viewer.shared_doc_title": "Shared document — click to manage"
+  "viewer.shared_doc_title": "Shared document — click to manage",
+  "dashboard.stats_files": "Files",
+  "dashboard.stats_size": "Total size",
+  "dashboard.stats_tags": "Unique tags",
+  "dashboard.stats_vaults": "Vaults",
+  "help.shortcuts_title": "Shortcuts & Tips"
 }