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/uf_header.dart

@@ -31,7 +31,7 @@ class UFHeader extends StatelessWidget {
       padding: const EdgeInsets.all(SpacingTokens.lg),
       decoration: BoxDecoration(
         gradient: const LinearGradient(
-          colors: [AppColors.primaryGreen, AppColors.brandGreenLight],
+          colors: [AppColors.primary, AppColors.primaryLight],
         ),
         borderRadius: BorderRadius.circular(SpacingTokens.radiusMd),
       ),