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_event_row.dart

@@ -1,8 +1,9 @@
 import 'package:flutter/material.dart';
+import '../tokens/app_colors.dart';
 import '../tokens/unionflow_colors.dart';
 
 /// Ligne d'événement à venir — style fintech avec bordure gauche verte
-/// Titre + date + countdown optionnel + participants optionnel
+/// Adaptatif dark/light via AppColors
 class DashboardEventRow extends StatelessWidget {
   final String title;
   final String date;
@@ -19,11 +20,17 @@ class DashboardEventRow extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final isDark       = Theme.of(context).brightness == Brightness.dark;
+    final bgColor      = isDark ? AppColors.surfaceDark      : AppColors.surface;
+    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: const Border(
           left: BorderSide(color: UnionFlowColors.unionGreen, width: 3),
@@ -37,21 +44,14 @@ class DashboardEventRow 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,
                 ),
                 const SizedBox(height: 2),
                 Text(
                   date,
-                  style: const TextStyle(
-                    fontSize: 10,
-                    color: UnionFlowColors.textSecondary,
-                  ),
+                  style: TextStyle(fontSize: 10, color: textSecondary),
                 ),
               ],
             ),
@@ -73,10 +73,7 @@ class DashboardEventRow extends StatelessWidget {
                   const SizedBox(height: 2),
                   Text(
                     participants!,
-                    style: const TextStyle(
-                      fontSize: 10,
-                      color: UnionFlowColors.textTertiary,
-                    ),
+                    style: TextStyle(fontSize: 10, color: textTertiary),
                   ),
                 ],
               ],