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/union_balance_card.dart

@@ -1,7 +1,9 @@
 import 'package:flutter/material.dart';
+import '../tokens/app_colors.dart';
 import '../tokens/unionflow_colors.dart';
 
-/// Card de balance UnionFlow - Affichage élégant du solde principal
+/// Card de balance UnionFlow — Affichage élégant du solde principal
+/// Adaptatif dark/light via AppColors
 class UnionBalanceCard extends StatelessWidget {
   final String label;
   final String amount;
@@ -20,14 +22,20 @@ class UnionBalanceCard 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;
+
     return GestureDetector(
       onTap: onTap,
       child: ClipRRect(
         borderRadius: BorderRadius.circular(10),
         child: Container(
           decoration: BoxDecoration(
-            color: UnionFlowColors.surface,
-            border: Border.all(color: UnionFlowColors.border),
+            color: bgColor,
+            border: Border.all(color: borderColor),
           ),
           child: Column(
             mainAxisSize: MainAxisSize.min,
@@ -43,20 +51,20 @@ class UnionBalanceCard extends StatelessWidget {
                         children: [
                           Text(
                             label.toUpperCase(),
-                            style: const TextStyle(
+                            style: TextStyle(
                               fontSize: 9,
                               fontWeight: FontWeight.w600,
-                              color: UnionFlowColors.textSecondary,
+                              color: textSecondary,
                               letterSpacing: 0.8,
                             ),
                           ),
                           const SizedBox(height: 4),
                           Text(
                             amount,
-                            style: const TextStyle(
+                            style: TextStyle(
                               fontSize: 22,
                               fontWeight: FontWeight.w800,
-                              color: UnionFlowColors.textPrimary,
+                              color: textPrimary,
                               height: 1,
                               letterSpacing: -0.5,
                             ),
@@ -76,8 +84,8 @@ class UnionBalanceCard extends StatelessWidget {
                                     : Icons.arrow_downward_rounded,
                                 size: 11,
                                 color: isTrendPositive == true
-                                    ? UnionFlowColors.success
-                                    : UnionFlowColors.error,
+                                    ? AppColors.success
+                                    : AppColors.error,
                               ),
                               const SizedBox(width: 2),
                               Text(
@@ -86,15 +94,15 @@ class UnionBalanceCard extends StatelessWidget {
                                   fontSize: 12,
                                   fontWeight: FontWeight.w700,
                                   color: isTrendPositive == true
-                                      ? UnionFlowColors.success
-                                      : UnionFlowColors.error,
+                                      ? AppColors.success
+                                      : AppColors.error,
                                 ),
                               ),
                             ],
                           ),
-                          const Text(
+                          Text(
                             'ce mois',
-                            style: TextStyle(fontSize: 9, color: UnionFlowColors.textTertiary),
+                            style: TextStyle(fontSize: 9, color: textSecondary),
                           ),
                         ],
                       ),