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/finance_workflow/presentation/widgets/reject_dialog.dart

@@ -53,6 +53,7 @@ class _RejectDialogState extends State<RejectDialog> {
 
   @override
   Widget build(BuildContext context) {
+    final isDark = Theme.of(context).brightness == Brightness.dark;
     final currencyFormat = NumberFormat.currency(symbol: widget.approval.currency);
 
     return AlertDialog(
@@ -75,9 +76,9 @@ class _RejectDialogState extends State<RejectDialog> {
               Container(
                 padding: const EdgeInsets.all(SpacingTokens.md),
                 decoration: BoxDecoration(
-                  color: AppColors.lightBackground,
+                  color: isDark ? AppColors.surfaceDark : AppColors.surface,
                   borderRadius: BorderRadius.circular(SpacingTokens.radiusSm),
-                  border: Border.all(color: AppColors.lightBorder),
+                  border: Border.all(color: isDark ? AppColors.borderDark : AppColors.border),
                 ),
                 child: Column(
                   crossAxisAlignment: CrossAxisAlignment.start,
@@ -91,7 +92,7 @@ class _RejectDialogState extends State<RejectDialog> {
                       'Montant',
                       currencyFormat.format(widget.approval.amount),
                       valueStyle: AppTypography.actionText.copyWith(
-                        color: AppColors.primaryGreen,
+                        color: AppColors.primary,
                         fontWeight: FontWeight.bold,
                       ),
                     ),
@@ -141,7 +142,7 @@ class _RejectDialogState extends State<RejectDialog> {
           label: const Text('Rejeter'),
           style: ElevatedButton.styleFrom(
             backgroundColor: AppColors.error,
-            foregroundColor: Colors.white,
+            foregroundColor: AppColors.onError,
           ),
         ),
       ],