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/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."
 }