Format resolution_date as YYYY-MM-DD in compliance table

Normalize the date in groupByHostname() to handle PostgreSQL Date objects,
and add .slice(0,10) in the frontend render as a safety net. Prevents the
full ISO timestamp (2026-05-15T00:00:00.000Z) from displaying in the table.

frontend/src/components/pages/CompliancePage.js

@@ -881,7 +881,7 @@ function DeviceRow({ device, selected, onClick }) {
 
             {/* Resolution Date */}
             <div style={{ fontFamily: 'monospace', fontSize: '0.72rem', color: '#94A3B8' }}>
-                {device.resolution_date || '—'}
+                {device.resolution_date ? device.resolution_date.slice(0, 10) : '—'}
             </div>
 
             {/* Remediation Plan */}