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_container.dart

@@ -125,7 +125,7 @@ class UFContainer extends StatelessWidget {
       alignment: alignment,
       constraints: constraints,
       decoration: BoxDecoration(
-        color: gradient == null ? (color ?? AppColors.lightSurface) : null,
+        color: gradient == null ? (color ?? Theme.of(context).colorScheme.surface) : null,
         gradient: gradient,
         borderRadius: BorderRadius.circular(borderRadius),
         border: border,