ed7ab61b90
`BackupHistory.jsx` opened `/admin/backup/download/<id>` via window.open, which goes to the React SPA's router — no matching route, so it rendered the "Page Not Found" screen. The actual download endpoint lives at `/api/admin/backup/download/:id` on the backend (adminBackup.js:685). Cookie-based admin auth already supports the implicit cookie sent by window.open, so the URL prefix was the only thing missing. Predates today's backup-hardening work — the bug has existed since this download button shipped. Surfaced now because Ralf finally has a completed backup to try downloading after the Stage A inline-dump guard started working.