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/dashboard/ProductivityAnalytics.tsx

@@ -110,7 +110,7 @@ export function ProductivityAnalytics({ metrics, deadlineMetrics, tagMetrics, se
         <PriorityDistributionChart data={filteredMetrics.priorityDistribution} />
         
         {/* Status Flow - Graphique simple en barres horizontales */}
-        <Card className="p-6">
+        <Card variant="glass" className="p-6">
           <h3 className="text-lg font-semibold mb-4">Répartition par Statut</h3>
           <div className="space-y-3">
             {filteredMetrics.statusFlow.map((item, index) => (
@@ -140,7 +140,7 @@ export function ProductivityAnalytics({ metrics, deadlineMetrics, tagMetrics, se
       <TagDistributionChart metrics={tagMetrics} />
 
       {/* Insights automatiques */}
-      <Card className="p-6">
+      <Card variant="glass" className="p-6">
         <h3 className="text-lg font-semibold mb-4">💡 Insights</h3>
         <div className="grid grid-cols-1 md:grid-cols-3 gap-4">
           <MetricCard