feat(i18n): add unknown sender/receiver fallback labels for document grouping

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

frontend/messages/en.json

@@ -80,6 +80,8 @@
 	"docs_empty_heading": "No documents found",
 	"docs_empty_text": "Try adjusting the filters or changing the search term.",
 	"docs_empty_btn_clear": "Clear all filters",
+	"docs_group_unknown_sender": "Unknown sender",
+	"docs_group_unknown_receiver": "Unknown recipient",
 	"docs_list_from": "From",
 	"docs_list_to": "To",
 	"docs_list_content": "Content",