Implement tender workflows and dashboard integration

lib/core/network/network_manager.dart

@@ -179,7 +179,16 @@ class NetworkManager {
   /// Handle HTTP error responses
   Result<T> _handleHttpError<T>(Response response) {
     final statusCode = response.statusCode;
-    final message = _extractErrorMessage(response.data);
+    var message = _extractErrorMessage(response.data);
+    if (statusCode == 422 && response.data is Map<String, dynamic>) {
+      final envelope = response.data as Map<String, dynamic>;
+      final error = envelope['error'];
+      if (error is Map<String, dynamic> &&
+          error['details'] is String &&
+          (error['details'] as String).isNotEmpty) {
+        message = error['details'] as String;
+      }
+    }
 
     switch (statusCode) {
       case 400:
@@ -238,16 +247,10 @@ class NetworkManager {
         );
 
       case DioExceptionType.badResponse:
-        if (e.response?.statusCode == 401) {
-          // Handle token refresh logic here if needed
-          // For now, return authentication error
-          return Result.error(
-            AuthenticationException(
-              'Authentication failed',
-              e.response?.statusCode,
-              e,
-            ),
-          );
+        if (e.response != null) {
+          // Preserve the backend envelope (including validation details) for
+          // non-2xx responses such as 409 and 422.
+          return _handleHttpError(e.response!);
         }
         return Result.error(
           FetchDataException(