fix: correct date formatting in VelocityMetrics component

- Updated date formatting in the `VelocityMetrics` component to ensure proper conversion of `week.weekStart` to a Date object before calling `toLocaleDateString`. This change improves the reliability of date display in the dashboard.

components/dashboard/VelocityMetrics.tsx

@@ -91,7 +91,7 @@ export function VelocityMetrics({ velocity }: VelocityMetricsProps) {
                     {week.totalActivities}
                   </div>
                   <div className="text-xs text-[var(--muted-foreground)]">
-                    {week.weekStart.toLocaleDateString('fr-FR', { 
+                    {new Date(week.weekStart).toLocaleDateString('fr-FR', { 
                       day: 'numeric',
                       month: 'short' 
                     })}
@@ -117,7 +117,7 @@ export function VelocityMetrics({ velocity }: VelocityMetricsProps) {
                 >
                   <div className="flex items-center gap-3">
                     <span className="text-sm font-medium">
-                      Semaine du {week.weekStart.toLocaleDateString('fr-FR', { 
+                      Semaine du {new Date(week.weekStart).toLocaleDateString('fr-FR', { 
                         day: 'numeric',
                         month: 'short' 
                       })}