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/buttons/uf_secondary_button.dart

@@ -34,12 +34,12 @@ class UFSecondaryButton extends StatelessWidget {
       child: ElevatedButton(
         onPressed: isLoading ? null : onPressed,
         style: ElevatedButton.styleFrom(
-          backgroundColor: AppColors.brandGreen,
+          backgroundColor: AppColors.primaryDark,
           foregroundColor: Colors.white,
-          disabledBackgroundColor: AppColors.brandGreen.withOpacity(0.5),
+          disabledBackgroundColor: AppColors.primaryDark.withOpacity(0.5),
           disabledForegroundColor: Colors.white.withOpacity(0.7),
           elevation: SpacingTokens.elevationSm,
-          shadowColor: AppColors.darkBorder.withOpacity(0.1),
+          shadowColor: AppColors.borderDark.withOpacity(0.1),
           padding: const EdgeInsets.symmetric(
             horizontal: SpacingTokens.buttonPaddingHorizontal,
             vertical: 10,