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/widgets/dynamic_fab.dart

@@ -21,7 +21,7 @@ class DynamicFAB extends StatelessWidget {
     if (label != null) {
       return FloatingActionButton.extended(
         onPressed: onPressed,
-        backgroundColor: AppColors.primaryGreen,
+        backgroundColor: AppColors.primary,
         foregroundColor: Colors.white,
         elevation: 4,
         icon: Icon(icon, size: 20),
@@ -34,7 +34,7 @@ class DynamicFAB extends StatelessWidget {
 
     return FloatingActionButton(
       onPressed: onPressed,
-      backgroundColor: AppColors.primaryGreen,
+      backgroundColor: AppColors.primary,
       foregroundColor: Colors.white,
       elevation: 4,
       child: Icon(icon, size: 24),