refactor: Enhance file attachment handling and UI components

- Updated the file attachment service to utilize a new LocalAttachment class, improving the management of file metadata such as display names.
- Refactored methods for picking and uploading files to accommodate the new LocalAttachment structure, ensuring consistent handling of file attributes.
- Improved the chat page to validate and manage file attachments more effectively, enhancing user experience during file uploads.
- Added functionality for image previews in the file attachment widget, allowing users to see selected images before sending.
- Introduced a remove button for attachments, improving usability by enabling users to easily discard unwanted files.

lib/shared/theme/tweakcn_themes.dart

@@ -482,7 +482,7 @@ class TweakcnThemes {
     preview: const <Color>[
       Color(0xFFC96442),
       Color(0xFFE9E6DC),
-      Color(0xFF9C87F5),
+      Color(0xFF1A1915),
     ],
   );
 
@@ -508,7 +508,7 @@ class TweakcnThemes {
     preview: const <Color>[
       Color(0xFFA1A1AA),
       Color(0xFFF4F4F5),
-      Color(0xFF2563EB),
+      Color(0xFF404040),
     ],
   );