style: update color theming in Kanban components

- Replaced hardcoded colors with CSS variables in ObjectivesBoard and TaskCard for improved theme consistency.
- Updated status icon in STATUS_CONFIG for better visual representation.
- Adjusted global CSS variables for enhanced contrast and clarity across components.

components/kanban/TaskCard.tsx

@@ -291,7 +291,7 @@ export function TaskCard({ task, onDelete, onEdit, onUpdateTitle, compactView =
           <div className="flex items-center justify-between text-xs">
             {task.dueDate ? (
               <span className="flex items-center gap-1 text-[var(--muted-foreground)] font-mono">
-                <span className="text-cyan-400">⏰</span>
+                <span className="text-[var(--primary)]">⏰</span>
                 {formatDistanceToNow(new Date(task.dueDate), { 
                   addSuffix: true, 
                   locale: fr