feat: update Card components to use variant="glass"

- Changed Card components in various charts and dashboard sections to use the "glass" variant for a consistent UI enhancement.
- This update affects CompletionTrendChart, PriorityDistributionChart, VelocityChart, WeeklyStatsCard, DashboardStats, ProductivityAnalytics, RecentTasks, TagDistributionChart, MetricsDistributionCharts, MetricsMainCharts, CriticalDeadlinesCard, DeadlineRiskCard, DeadlineSummaryCard, and StatCard.

src/components/charts/CompletionTrendChart.tsx

@@ -49,7 +49,7 @@ export function CompletionTrendChart({ data, title = "Tendance de Completion" }:
   };
 
   return (
-    <Card className="p-6">
+    <Card variant="glass" className="p-6">
       <h3 className="text-lg font-semibold mb-4">{title}</h3>
       <div className="h-64">
         <ResponsiveContainer width="100%" height="100%">