feat(UI): Implement holiday awareness across all data widgets

- All data widgets (tables and cards) now use the useIsHoliday hook.
- An informational alert is displayed on holidays, without hiding the last available data.
- Unifies loading state logic to wait for both data and holiday status, preventing race conditions.
- Ensures a consistent and robust user experience across all components."

frontend/src/components/MervalHeroCard.tsx

@@ -90,7 +90,7 @@ export const MervalHeroCard = () => {
             )}
 
             {/* El contenido principal del widget siempre se muestra si hay datos. */}
-            <Paper elevation={3} sx={{ p: 2 }}>
+            <Paper elevation={3} sx={{ p: 2, mb: 3 }}>
                 <Box sx={{ display: 'flex', justifyContent: 'space-between', alignItems: 'flex-start', mb: 2 }}>
                     <Box>
                         <Typography variant="h6" component="h2" sx={{ fontWeight: 'bold' }}>Índice S&P MERVAL</Typography>