Add Backups tab to AdminMonitor, backup_path label to Settings

Backups tab shows last-run status, expandable run history with per-item
breakdown (db/vol, size, status), and a Run Now trigger button.
Settings Nextcloud card gains a Backup Directory field.

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

src/pages/AdminSettings.tsx

@@ -34,6 +34,7 @@ const FIELD_LABELS: Record<string, string> = {
   graphql_endpoint: 'GraphQL Endpoint', bearer_token: 'Bearer Token',
   email: 'Email', sync_hours: 'Sync Interval (hours)', user: 'Username', pass: 'Password',
   location_id: 'Default Location', from: 'From Address', reply_to: 'Reply-To Address',
+  backup_path: 'Backup Directory',
 }
 
 interface AppRow {