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/widgets/file_upload_widget.dart

@@ -2,6 +2,7 @@ import 'dart:io';
 import 'package:flutter/material.dart';
 import 'package:file_picker/file_picker.dart';
 import 'package:image_picker/image_picker.dart';
+import '../design_system/tokens/app_colors.dart';
 
 /// Widget réutilisable pour uploader un fichier (image ou PDF)
 /// avec prévisualisation et validation
@@ -105,7 +106,7 @@ class _FileUploadWidgetState extends State<FileUploadWidget> {
       ScaffoldMessenger.of(context).showSnackBar(
         SnackBar(
           content: Text(message),
-          backgroundColor: Colors.red,
+          backgroundColor: AppColors.error,
         ),
       );
     }
@@ -179,7 +180,7 @@ class _FileUploadWidgetState extends State<FileUploadWidget> {
                 ),
                 if (_selectedFile != null)
                   IconButton(
-                    icon: const Icon(Icons.delete, color: Colors.red),
+                    icon: const Icon(Icons.delete, color: AppColors.error),
                     onPressed: _removeFile,
                   ),
               ],
@@ -202,7 +203,7 @@ class _FileUploadWidgetState extends State<FileUploadWidget> {
                   width: double.infinity,
                   decoration: BoxDecoration(
                     borderRadius: BorderRadius.circular(8),
-                    border: Border.all(color: Colors.grey.shade300),
+                    border: Border.all(color: AppColors.borderStrong),
                   ),
                   child: ClipRRect(
                     borderRadius: BorderRadius.circular(8),
@@ -218,13 +219,13 @@ class _FileUploadWidgetState extends State<FileUploadWidget> {
                   width: double.infinity,
                   decoration: BoxDecoration(
                     borderRadius: BorderRadius.circular(8),
-                    border: Border.all(color: Colors.grey.shade300),
+                    border: Border.all(color: AppColors.borderStrong),
                   ),
                   child: const Center(
                     child: Column(
                       mainAxisAlignment: MainAxisAlignment.center,
                       children: [
-                        Icon(Icons.picture_as_pdf, size: 48, color: Colors.red),
+                        Icon(Icons.picture_as_pdf, size: 48, color: AppColors.error),
                         SizedBox(height: 8),
                         Text('Document PDF'),
                       ],
@@ -234,7 +235,11 @@ class _FileUploadWidgetState extends State<FileUploadWidget> {
               const SizedBox(height: 8),
               Text(
                 _selectedFile!.path.split('/').last,
-                style: TextStyle(color: Colors.grey.shade700),
+                style: TextStyle(
+                  color: Theme.of(context).brightness == Brightness.dark
+                      ? AppColors.textSecondaryDark
+                      : AppColors.textSecondary,
+                ),
                 maxLines: 1,
                 overflow: TextOverflow.ellipsis,
               ),
@@ -244,7 +249,7 @@ class _FileUploadWidgetState extends State<FileUploadWidget> {
               'Formats acceptés: JPEG, PNG, PDF (max 5 MB)',
               style: TextStyle(
                 fontSize: 12,
-                color: Colors.grey.shade600,
+                color: AppColors.textTertiary,
               ),
             ),
           ],