fix(i18n): replace hardcoded strings with Paraglide message keys

- error_file_upload_failed key used in enrich upload handler
- label_optional key added (de/en/es) and used in DescriptionSection divider

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

frontend/messages/de.json

@@ -52,6 +52,7 @@
 	"form_label_archive_location": "Aufbewahrungsort",
 	"form_placeholder_archive_location": "z.B. Schrank 3, Mappe B",
 	"form_helper_archive_location": "Wo befindet sich das Originaldokument?",
+	"label_optional": "Optional",
 	"login_heading": "Anmelden",
 	"login_label_username": "Benutzername",
 	"login_label_password": "Passwort",

frontend/messages/en.json

@@ -52,6 +52,7 @@
 	"form_label_archive_location": "Storage location",
 	"form_placeholder_archive_location": "e.g. Cabinet 3, Folder B",
 	"form_helper_archive_location": "Where is the original document stored?",
+	"label_optional": "Optional",
 	"login_heading": "Sign in",
 	"login_label_username": "Username",
 	"login_label_password": "Password",

frontend/messages/es.json

@@ -52,6 +52,7 @@
 	"form_label_archive_location": "Ubicación de almacenamiento",
 	"form_placeholder_archive_location": "p.ej. Armario 3, Carpeta B",
 	"form_helper_archive_location": "¿Dónde se encuentra el documento original?",
+	"label_optional": "Opcional",
 	"login_heading": "Iniciar sesión",
 	"login_label_username": "Usuario",
 	"login_label_password": "Contraseña",