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/connected/connected_upcoming_events.dart

@@ -30,14 +30,14 @@ class ConnectedUpcomingEvents extends StatelessWidget {
               if (state is DashboardLoading) {
                 return _buildLoadingList();
               } else if (state is DashboardLoaded || state is DashboardRefreshing) {
-                final data = state is DashboardLoaded 
-                    ? state.dashboardData 
+                final data = state is DashboardLoaded
+                    ? state.dashboardData
                     : (state as DashboardRefreshing).dashboardData;
                 return _buildEventsList(context, data.upcomingEvents);
               } else if (state is DashboardError) {
                 return _buildErrorState(state.message);
               }
-              return _buildEmptyState();
+              return _buildEmptyState(context);
             },
           ),
         ],
@@ -50,7 +50,7 @@ class ConnectedUpcomingEvents extends StatelessWidget {
       children: [
         const Icon(
           Icons.event_outlined,
-          color: AppColors.primaryGreen,
+          color: AppColors.primary,
           size: 18,
         ),
         const SizedBox(width: 8),
@@ -66,7 +66,7 @@ class ConnectedUpcomingEvents extends StatelessWidget {
             child: Text(
               'TOUT VOIR',
               style: AppTypography.badgeText.copyWith(
-                color: AppColors.primaryGreen,
+                color: AppColors.primary,
                 fontWeight: FontWeight.bold,
               ),
             ),
@@ -77,7 +77,7 @@ class ConnectedUpcomingEvents extends StatelessWidget {
 
   Widget _buildEventsList(BuildContext context, List<UpcomingEventEntity> events) {
     if (events.isEmpty) {
-      return _buildEmptyState();
+      return _buildEmptyState(context);
     }
 
     final displayEvents = events.take(maxItems).toList();
@@ -99,7 +99,7 @@ class ConnectedUpcomingEvents extends StatelessWidget {
   }
 
   Widget _buildEventCard(BuildContext context, UpcomingEventEntity event) {
-    final statusColor = event.isToday ? AppColors.success : (event.isTomorrow ? AppColors.warning : AppColors.primaryGreen);
+    final statusColor = event.isToday ? AppColors.success : (event.isTomorrow ? AppColors.warning : AppColors.primary);
 
     return CoreCard(
       backgroundColor: Theme.of(context).cardColor,
@@ -140,7 +140,7 @@ class ConnectedUpcomingEvents extends StatelessWidget {
                     ),
                     Row(
                       children: [
-                        const Icon(Icons.location_on_outlined, size: 10, color: AppColors.textSecondaryLight),
+                        Icon(Icons.location_on_outlined, size: 10, color: Theme.of(context).colorScheme.onSurfaceVariant),
                         const SizedBox(width: 4),
                         Expanded(
                           child: Text(
@@ -188,7 +188,7 @@ class ConnectedUpcomingEvents extends StatelessWidget {
                 child: LinearProgressIndicator(
                   value: event.fillPercentage,
                   minHeight: 4,
-                  backgroundColor: AppColors.lightBorder,
+                  backgroundColor: Theme.of(context).brightness == Brightness.dark ? AppColors.borderDark : AppColors.border,
                   valueColor: AlwaysStoppedAnimation<Color>(
                     event.isFull ? AppColors.error : (event.isAlmostFull ? AppColors.warning : AppColors.success),
                   ),
@@ -230,11 +230,12 @@ class ConnectedUpcomingEvents extends StatelessWidget {
     );
   }
 
-  Widget _buildEmptyState() {
+  Widget _buildEmptyState(BuildContext context) {
+    final isDark = Theme.of(context).brightness == Brightness.dark;
     return Center(
       child: Column(
         children: [
-          const Icon(Icons.event_outlined, color: AppColors.textSecondaryLight, size: 32),
+          Icon(Icons.event_outlined, color: isDark ? AppColors.textSecondaryDark : AppColors.textSecondary, size: 32),
           const SizedBox(height: 8),
           const Text('AUCUN ÉVÉNEMENT', style: AppTypography.subtitleSmall),
           Text('Les événements apparaîtront ici', style: AppTypography.subtitleSmall.copyWith(fontSize: 10)),