feat(shared): legacy presentation/ + shared design system + widgets

- lib/presentation : pages legacy (explore/network, notifications) avec BLoC
- lib/shared/design_system : UnionFlow Design System v2 (tokens, components)
  + MD3 tokens + module_colors par feature
- lib/shared/widgets : widgets transversaux (core_card, core_shimmer,
  error_widget, loading_widget, powered_by_lions_dev, etc.)
- lib/shared/constants + utils

lib/shared/design_system/components/cards/uf_card.dart

@@ -89,7 +89,7 @@ class UFCard extends StatelessWidget {
 
     Widget content = Container(
       padding: effectivePadding,
-      decoration: _getDecoration(effectiveBorderRadius),
+      decoration: _getDecoration(effectiveBorderRadius, context),
       child: child,
     );
 
@@ -108,27 +108,31 @@ class UFCard extends StatelessWidget {
     );
   }
 
-  BoxDecoration _getDecoration(double radius) {
+  BoxDecoration _getDecoration(double radius, BuildContext context) {
+    final surfaceColor = Theme.of(context).colorScheme.surface;
+    final isDark = Theme.of(context).brightness == Brightness.dark;
+    final effectiveBorderColor = borderColor ?? (isDark ? AppColors.borderDark : AppColors.border);
+
     switch (style) {
       case UFCardStyle.elevated:
         return BoxDecoration(
-          color: color ?? AppColors.lightSurface,
+          color: color ?? surfaceColor,
           borderRadius: BorderRadius.circular(radius),
         );
 
       case UFCardStyle.outlined:
         return BoxDecoration(
-          color: color ?? AppColors.lightSurface,
+          color: color ?? surfaceColor,
           borderRadius: BorderRadius.circular(radius),
           border: Border.all(
-            color: borderColor ?? AppColors.lightBorder,
+            color: effectiveBorderColor,
             width: borderWidth ?? 1.0,
           ),
         );
 
       case UFCardStyle.filled:
         return BoxDecoration(
-          color: color ?? AppColors.lightSurface,
+          color: color ?? surfaceColor,
           borderRadius: BorderRadius.circular(radius),
         );
     }