fix(monitoring): read used_mem_mb from nested system object in API response

The /api/system/info response wraps SystemInfo under data.system,
not directly under data.

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

controller/internal/web/templates/monitoring.html

@@ -532,7 +532,7 @@
         try {
             var resp = await fetch('/api/system/info');
             var json = await resp.json();
-            if (json.ok && json.data) usedMB = json.data.used_mem_mb || 0;
+            if (json.ok && json.data && json.data.system) usedMB = json.data.system.used_mem_mb || 0;
         } catch(e) {}
         if (!usedMB) return;