fix(disk-monitoring): use correct admin_token from localStorage

Fix authentication error in disk monitoring dashboard:
- Change from 'token' to 'admin_token' for consistency with admin system
- Add check for missing token with user-friendly error message
- Prevents 401 errors when admin is logged in

Fixes: Failed to fetch metrics: 401 (Unauthorized)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

public/js/admin-disk-monitoring.js

@@ -11,7 +11,11 @@ async function loadMetrics() {
     metricsContainer.classList.add('hidden');
     errorDiv.classList.add('hidden');
 
-    const token = localStorage.getItem('token');
+    const token = localStorage.getItem('admin_token');
+    if (!token) {
+      throw new Error('Not authenticated. Please log in.');
+    }
+
     const response = await fetch('/api/admin/disk-metrics', {
       headers: { 'Authorization': 'Bearer ' + token }
     });