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.

lib/status-config.ts

@@ -19,7 +19,7 @@ export const STATUS_CONFIG: Record<TaskStatus, StatusConfig> = {
   in_progress: {
     key: 'in_progress',
     label: 'En cours',
-    icon: '🔄',
+    icon: '⚙️',
     color: 'blue',
     order: 2
   },