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/core_card.dart

@@ -1,5 +1,4 @@
 import 'package:flutter/material.dart';
-import '../design_system/tokens/app_colors.dart';
 
 /// UnionFlow Mobile - Composant DRY Centralisé : CoreCard
 /// Le seul et unique conteneur d'affichage (Posts, Événements, Profils).
@@ -22,21 +21,21 @@ class CoreCard extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final isDark = Theme.of(context).brightness == Brightness.dark;
+    final scheme = Theme.of(context).colorScheme;
 
     return Container(
       width: double.infinity,
       margin: margin,
       decoration: BoxDecoration(
-        color: backgroundColor ?? (isDark ? AppColors.darkSurface : Colors.white),
+        color: backgroundColor ?? scheme.surface,
         borderRadius: BorderRadius.circular(10.0),
         border: Border.all(
-          color: isDark ? AppColors.darkBorder.withOpacity(0.5) : AppColors.lightBorder,
+          color: scheme.outlineVariant.withOpacity(0.6),
           width: 0.8,
         ),
         boxShadow: [
           BoxShadow(
-            color: Colors.black.withOpacity(isDark ? 0.15 : 0.04),
+            color: scheme.shadow.withOpacity(0.06),
             blurRadius: 6,
             offset: const Offset(0, 2),
           ),