feat(image): Improve image attachment loading and error handling

lib/features/chat/utils/file_utils.dart

@@ -0,0 +1,22 @@
+// Utility functions for handling file data in chat messages.
+// Used by both user and assistant message widgets.
+
+/// Checks if a file map represents an image.
+/// Matches OpenWebUI behavior: type === 'image' OR content_type starts with 'image/'
+bool isImageFile(dynamic file) {
+  if (file is! Map) return false;
+  if (file['type'] == 'image') return true;
+  final contentType = file['content_type']?.toString() ?? '';
+  return contentType.startsWith('image/');
+}
+
+/// Extracts the file URL or ID from a file map.
+/// OpenWebUI stores either a full URL, data URL, or just the file ID.
+/// 
+/// Returns the URL/ID string, or null if the file has no valid URL.
+String? getFileUrl(dynamic file) {
+  if (file is! Map) return null;
+  final url = file['url'];
+  if (url == null) return null;
+  return url.toString();
+}