feat(frontend): add System tab to admin panel with backfill-versions action

Admin can trigger an initial history snapshot for all documents without
version history. Shows count of backfilled documents after completion.

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

frontend/messages/de.json

@@ -233,5 +233,10 @@
 	"history_field_summary": "Zusammenfassung",
 	"history_field_sender": "Absender",
 	"history_field_receivers": "Empfänger",
-	"history_field_tags": "Schlagworte"
+	"history_field_tags": "Schlagworte",
+	"admin_tab_system": "System",
+	"admin_system_backfill_heading": "Verlaufsdaten auffüllen",
+	"admin_system_backfill_description": "Erstellt einen initialen Verlaufseintrag für alle Dokumente, die noch keinen Verlauf haben (z.B. importierte Dokumente). Dadurch werden beim nächsten Bearbeiten nur die tatsächlich geänderten Felder hervorgehoben.",
+	"admin_system_backfill_btn": "Jetzt auffüllen",
+	"admin_system_backfill_success": "{count} Dokumente wurden aufgefüllt."
 }

frontend/messages/en.json

@@ -233,5 +233,10 @@
 	"history_field_summary": "Summary",
 	"history_field_sender": "Sender",
 	"history_field_receivers": "Receivers",
-	"history_field_tags": "Tags"
+	"history_field_tags": "Tags",
+	"admin_tab_system": "System",
+	"admin_system_backfill_heading": "Backfill history data",
+	"admin_system_backfill_description": "Creates an initial history entry for all documents that do not have one yet (e.g. imported documents). This ensures that future edits only highlight actually changed fields.",
+	"admin_system_backfill_btn": "Backfill now",
+	"admin_system_backfill_success": "{count} documents were backfilled."
 }

frontend/messages/es.json

@@ -233,5 +233,10 @@
 	"history_field_summary": "Resumen",
 	"history_field_sender": "Remitente",
 	"history_field_receivers": "Destinatarios",
-	"history_field_tags": "Etiquetas"
+	"history_field_tags": "Etiquetas",
+	"admin_tab_system": "Sistema",
+	"admin_system_backfill_heading": "Completar datos de historial",
+	"admin_system_backfill_description": "Crea una entrada de historial inicial para todos los documentos que aún no tienen ninguna (p.ej. documentos importados). Así, en la próxima edición solo se resaltarán los campos realmente modificados.",
+	"admin_system_backfill_btn": "Completar ahora",
+	"admin_system_backfill_success": "{count} documentos fueron completados."
 }