feat(i18n): add btn_confirm key to de/en/es message files

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -21,6 +21,7 @@
 	"nav_logout": "Abmelden",
 	"btn_save": "Speichern",
 	"btn_cancel": "Abbrechen",
+	"btn_confirm": "Bestätigen",
 	"btn_edit": "Bearbeiten",
 	"btn_create": "Erstellen",
 	"btn_delete": "Löschen",