feat(design-system): dark mode adaptatif sur widgets partagés

Pattern AppColors pair (isDark ? AppColors.surfaceDark : AppColors.surface)
appliqué sur :
- UnionStatWidget, UnionBalanceCard, UnionActionButton, UFSectionHeader
- DashboardEventRow, DashboardActivityRow, UnionExportButton
- MiniAvatar (border adaptatif)
- ConfirmationDialog (cancel colors adaptés)
- FileUploadWidget (textSecondary adaptatif)

Les couleurs surface/border/textPrimary/textSecondary hardcodées (light-only)
sont remplacées par les paires *Dark conditionnelles.
Les couleurs sémantiques (error, success, warning, primary) restent inchangées.

lib/shared/design_system/components/dashboard_activity_row.dart

@@ -1,8 +1,9 @@
 import 'package:flutter/material.dart';
+import '../tokens/app_colors.dart';
 import '../tokens/unionflow_colors.dart';
 
-/// Ligne d'activité récente — style fintech compact identique au super_admin
-/// Icône dans carré arrondi 28×28 + titre + description + timestamp
+/// Ligne d'activité récente — style fintech compact
+/// Adaptatif dark/light via AppColors
 class DashboardActivityRow extends StatelessWidget {
   final String title;
   final String description;
@@ -21,13 +22,20 @@ class DashboardActivityRow extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final isDark        = Theme.of(context).brightness == Brightness.dark;
+    final bgColor       = isDark ? AppColors.surfaceDark      : AppColors.surface;
+    final borderColor   = isDark ? AppColors.borderDark       : AppColors.border;
+    final textPrimary   = isDark ? AppColors.textPrimaryDark  : AppColors.textPrimary;
+    final textSecondary = isDark ? AppColors.textSecondaryDark: AppColors.textSecondary;
+    final textTertiary  = isDark ? AppColors.textSecondaryDark: AppColors.textTertiary;
+
     return Container(
       margin: const EdgeInsets.only(bottom: 6),
       padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 9),
       decoration: BoxDecoration(
-        color: UnionFlowColors.surface,
+        color: bgColor,
         borderRadius: BorderRadius.circular(8),
-        border: Border.all(color: UnionFlowColors.border),
+        border: Border.all(color: borderColor),
       ),
       child: Row(
         children: [
@@ -35,7 +43,7 @@ class DashboardActivityRow extends StatelessWidget {
             width: 28,
             height: 28,
             decoration: BoxDecoration(
-              color: color.withOpacity(0.12),
+              color: color.withOpacity(isDark ? 0.2 : 0.12),
               borderRadius: BorderRadius.circular(6),
             ),
             child: Icon(icon, size: 14, color: color),
@@ -47,20 +55,13 @@ class DashboardActivityRow extends StatelessWidget {
               children: [
                 Text(
                   title,
-                  style: const TextStyle(
-                    fontSize: 12,
-                    fontWeight: FontWeight.w600,
-                    color: UnionFlowColors.textPrimary,
-                  ),
+                  style: TextStyle(fontSize: 12, fontWeight: FontWeight.w600, color: textPrimary),
                   maxLines: 1,
                   overflow: TextOverflow.ellipsis,
                 ),
                 Text(
                   description,
-                  style: const TextStyle(
-                    fontSize: 10,
-                    color: UnionFlowColors.textSecondary,
-                  ),
+                  style: TextStyle(fontSize: 10, color: textSecondary),
                   maxLines: 1,
                   overflow: TextOverflow.ellipsis,
                 ),
@@ -68,10 +69,7 @@ class DashboardActivityRow extends StatelessWidget {
             ),
           ),
           const SizedBox(width: 8),
-          Text(
-            timeAgo,
-            style: const TextStyle(fontSize: 10, color: UnionFlowColors.textTertiary),
-          ),
+          Text(timeAgo, style: TextStyle(fontSize: 10, color: textTertiary)),
         ],
       ),
     );
@@ -80,28 +78,20 @@ class DashboardActivityRow extends StatelessWidget {
   /// Icône selon le type d'activité
   static IconData iconFor(String type) {
     switch (type) {
-      case 'member':
-        return Icons.person_add_rounded;
-      case 'event':
-        return Icons.event_rounded;
-      case 'contribution':
-        return Icons.payments_rounded;
-      default:
-        return Icons.circle_notifications_rounded;
+      case 'member':       return Icons.person_add_rounded;
+      case 'event':        return Icons.event_rounded;
+      case 'contribution': return Icons.payments_rounded;
+      default:             return Icons.circle_notifications_rounded;
     }
   }
 
   /// Couleur selon le type d'activité
   static Color colorFor(String type) {
     switch (type) {
-      case 'member':
-        return UnionFlowColors.unionGreen;
-      case 'event':
-        return UnionFlowColors.info;
-      case 'contribution':
-        return UnionFlowColors.gold;
-      default:
-        return UnionFlowColors.textSecondary;
+      case 'member':       return UnionFlowColors.unionGreen;
+      case 'event':        return UnionFlowColors.info;
+      case 'contribution': return UnionFlowColors.gold;
+      default:             return AppColors.textSecondary;
     }
   }
 }