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.

src/app/globals.css

@@ -2,16 +2,16 @@
 
 :root {
   /* Dark theme (default) */
-  --background: #020617; /* slate-950 */
+  --background: #1e293b; /* slate-800 - encore plus clair */
   --foreground: #f1f5f9; /* slate-100 */
-  --card: #0f172a; /* slate-900 */
-  --card-hover: #1e293b; /* slate-800 */
-  --card-column: #0f172a; /* slate-900 - same as card in dark */
-  --border: #334155; /* slate-700 */
-  --input: #1e293b; /* slate-800 */
+  --card: #334155; /* slate-700 - beaucoup plus clair pour contraste fort */
+  --card-hover: #475569; /* slate-600 */
+  --card-column: #0f172a; /* slate-900 - plus foncé que les cartes */
+  --border: #64748b; /* slate-500 - encore plus clair */
+  --input: #334155; /* slate-700 - plus clair */
   --primary: #06b6d4; /* cyan-500 */
   --primary-foreground: #f1f5f9; /* slate-100 */
-  --muted: #475569; /* slate-600 */
+  --muted: #64748b; /* slate-500 */
   --muted-foreground: #94a3b8; /* slate-400 */
   --accent: #f59e0b; /* amber-500 */
   --destructive: #ef4444; /* red-500 */