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/inputs/uf_dropdown_tile.dart

@@ -50,7 +50,7 @@ class UFDropdownTile<T> extends StatelessWidget {
   Widget build(BuildContext context) {
     final isDark = Theme.of(context).brightness == Brightness.dark;
     final effectiveBgColor = backgroundColor ??
-        (isDark ? AppColors.darkSurface : AppColors.lightSurface);
+        (isDark ? AppColors.surfaceDark : AppColors.surface);
     final effectiveItemBuilder = itemBuilder ?? (item) => item.toString();
 
     return Container(
@@ -67,16 +67,16 @@ class UFDropdownTile<T> extends StatelessWidget {
               title,
               style: AppTypography.bodyTextSmall.copyWith(
                 fontWeight: FontWeight.w600,
-                color: isDark ? AppColors.textPrimaryDark : AppColors.textPrimaryLight,
+                color: isDark ? AppColors.textPrimaryDark : AppColors.textPrimary,
               ),
             ),
           ),
           Container(
             padding: const EdgeInsets.symmetric(horizontal: SpacingTokens.lg),
             decoration: BoxDecoration(
-              color: isDark ? AppColors.darkBackground : Colors.white,
+              color: isDark ? AppColors.backgroundDark : Colors.white,
               borderRadius: BorderRadius.circular(SpacingTokens.radiusMd),
-              border: Border.all(color: isDark ? AppColors.darkBorder : AppColors.lightBorder),
+              border: Border.all(color: isDark ? AppColors.borderDark : AppColors.border),
             ),
             child: DropdownButtonHideUnderline(
               child: DropdownButton<T>(