feat(ui): dark mode adaptatif sur 15 pages/widgets restants

Pattern AppColors pair (isDark ternaries) appliqué sur :
- login_page : SnackBar error color Color(0xFFDC2626) → AppColors.error
  (gradient brand intentionnel non modifié)
- help_support : barre de recherche + ExpansionTile + chevrons → scheme adaptatif
- system_settings : état 'Accès réservé' + unselectedLabelColor TabBar
- epargne : date/description/boutons OutlinedButton foregroundColor adaptatifs
- conversation_tile, connected_recent_activities, connected_upcoming_events
- dashboard_notifications_widget
- budgets_list_page, pending_approvals_page, approve/reject_dialog
- create_organization_page, edit_organization_page, about_page

Les couleurs sémantiques (error, success, warning, primary) restent inchangées.
Les blancs/gradients intentionnels (AppBars brand, logos payment) préservés.

lib/features/dashboard/presentation/widgets/notifications/dashboard_notifications_widget.dart

@@ -37,7 +37,7 @@ class DashboardNotificationsWidget extends StatelessWidget {
               } else if (state is DashboardError) {
                 return _buildErrorNotifications();
               }
-              return _buildEmptyNotifications();
+              return _buildEmptyNotifications(context);
             },
           ),
         ],
@@ -49,7 +49,7 @@ class DashboardNotificationsWidget extends StatelessWidget {
     return Container(
       padding: const EdgeInsets.all(12),
       decoration: BoxDecoration(
-        color: AppColors.primaryGreen.withOpacity(0.05),
+        color: AppColors.primary.withOpacity(0.05),
         borderRadius: const BorderRadius.only(
           topLeft: Radius.circular(12),
           topRight: Radius.circular(12),
@@ -60,7 +60,7 @@ class DashboardNotificationsWidget extends StatelessWidget {
           Container(
             padding: const EdgeInsets.all(6),
             decoration: BoxDecoration(
-              color: AppColors.primaryGreen,
+              color: AppColors.primary,
               borderRadius: BorderRadius.circular(4),
             ),
             child: const Icon(
@@ -74,7 +74,7 @@ class DashboardNotificationsWidget extends StatelessWidget {
             child: Text(
               'NOTIFICATIONS',
               style: AppTypography.subtitleSmall.copyWith(
-                color: AppColors.primaryGreen,
+                color: AppColors.primary,
                 fontWeight: FontWeight.bold,
                 letterSpacing: 1.1,
               ),
@@ -119,25 +119,26 @@ class DashboardNotificationsWidget extends StatelessWidget {
 
   Widget _buildNotifications(BuildContext context, DashboardEntity data) {
     final notifications = _generateNotifications(context, data);
-    
+
     if (notifications.isEmpty) {
-      return _buildEmptyNotifications();
+      return _buildEmptyNotifications(context);
     }
 
     return Column(
       children: notifications.take(maxNotifications).map((notification) {
-        return _buildNotificationItem(notification);
+        return _buildNotificationItem(context, notification);
       }).toList(),
     );
   }
 
-  Widget _buildNotificationItem(DashboardNotification notification) {
+  Widget _buildNotificationItem(BuildContext context, DashboardNotification notification) {
+    final isDark = Theme.of(context).brightness == Brightness.dark;
     return Container(
       padding: const EdgeInsets.all(12),
       decoration: BoxDecoration(
         border: Border(
           bottom: BorderSide(
-            color: AppColors.lightBorder,
+            color: isDark ? AppColors.borderDark : AppColors.border,
             width: 1,
           ),
         ),
@@ -199,7 +200,7 @@ class DashboardNotificationsWidget extends StatelessWidget {
                 Text(
                   notification.message,
                   style: AppTypography.subtitleSmall.copyWith(
-                    color: AppColors.textSecondaryLight,
+                    color: isDark ? AppColors.textSecondaryDark : AppColors.textSecondary,
                     fontSize: 10,
                   ),
                 ),
@@ -209,7 +210,7 @@ class DashboardNotificationsWidget extends StatelessWidget {
                     Text(
                       notification.timeAgo,
                       style: AppTypography.subtitleSmall.copyWith(
-                        color: AppColors.textSecondaryLight,
+                        color: isDark ? AppColors.textSecondaryDark : AppColors.textSecondary,
                         fontSize: 9,
                       ),
                     ),
@@ -220,7 +221,7 @@ class DashboardNotificationsWidget extends StatelessWidget {
                         child: Text(
                           notification.actionLabel!,
                           style: AppTypography.badgeText.copyWith(
-                            color: AppColors.primaryGreen,
+                            color: AppColors.primary,
                             fontWeight: FontWeight.bold,
                             fontSize: 9,
                           ),
@@ -268,15 +269,16 @@ class DashboardNotificationsWidget extends StatelessWidget {
     );
   }
 
-  Widget _buildEmptyNotifications() {
+  Widget _buildEmptyNotifications(BuildContext context) {
+    final isDark = Theme.of(context).brightness == Brightness.dark;
     return Container(
       padding: const EdgeInsets.all(24),
       child: Center(
         child: Column(
           children: [
-            const Icon(
+            Icon(
               Icons.notifications_none_outlined,
-              color: AppColors.textSecondaryLight,
+              color: isDark ? AppColors.textSecondaryDark : AppColors.textSecondary,
               size: 24,
             ),
             const SizedBox(height: 8),
@@ -359,7 +361,7 @@ class DashboardNotificationsWidget extends StatelessWidget {
         title: 'Nouvelles activités',
         message: '${data.recentActivitiesCount} activités récentes',
         icon: Icons.fiber_new_outlined,
-        color: AppColors.brandGreen,
+        color: AppColors.primaryDark,
         timeAgo: '15min',
         isUrgent: false,
         actionLabel: 'Voir',