Fix plus sign showing before negative rendimientos values

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/Pensions.tsx

@@ -607,7 +607,7 @@ export default function Pensions() {
                     </p>
                   )}
                   <p className="text-lg font-bold font-mono" style={{ color: fund.color }}>
-                    +{formatCRC(earned)}
+                    {earned >= 0 ? '+' : ''}{formatCRC(earned)}
                   </p>
                   <p className="text-xs text-muted-foreground">en rendimientos</p>
                 </CardContent>