feat(i18n): add sort, result count, and empty-state translation keys

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

frontend/messages/de.json

@@ -52,7 +52,15 @@
 	"login_label_username": "Benutzername",
 	"login_label_password": "Passwort",
 	"login_btn_submit": "Anmelden",
-	"docs_search_placeholder": "Suche in Titel, Inhalt, Ort...",
+	"docs_search_placeholder": "Titel, Personen, Tags durchsuchen…",
+	"docs_sort_label": "Sortierung",
+	"docs_sort_date": "Datum",
+	"docs_sort_title": "Titel",
+	"docs_sort_sender": "Absender",
+	"docs_sort_receiver": "Empfänger",
+	"docs_sort_upload": "Hochgeladen",
+	"docs_result_count": "{count} Dokumente",
+	"docs_empty_for_term": "Keine Dokumente f\u00fcr \"{term}\" gefunden",
 	"docs_btn_filter": "Filter",
 	"docs_btn_reset_title": "Filter zurücksetzen",
 	"docs_filter_label_tags": "Schlagworte",