i18n: add docs_group_undated and docs_group_unknown translation keys

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

frontend/messages/de.json

@@ -79,6 +79,8 @@
 	"docs_list_from": "Von",
 	"docs_list_to": "An",
 	"docs_list_unknown": "Unbekannt",
+	"docs_group_undated": "Undatiert",
+	"docs_group_unknown": "Unbekannt",
 	"doc_section_who_when": "Wer & Wann",
 	"doc_section_description": "Beschreibung",
 	"doc_section_file": "Datei",

frontend/messages/en.json

@@ -79,6 +79,8 @@
 	"docs_list_from": "From",
 	"docs_list_to": "To",
 	"docs_list_unknown": "Unknown",
+	"docs_group_undated": "Undated",
+	"docs_group_unknown": "Unknown",
 	"doc_section_who_when": "Who & When",
 	"doc_section_description": "Description",
 	"doc_section_file": "File",

frontend/messages/es.json

@@ -79,6 +79,8 @@
 	"docs_list_from": "De",
 	"docs_list_to": "Para",
 	"docs_list_unknown": "Desconocido",
+	"docs_group_undated": "Sin fecha",
+	"docs_group_unknown": "Desconocido",
 	"doc_section_who_when": "Quién & Cuándo",
 	"doc_section_description": "Descripción",
 	"doc_section_file": "Archivo",