feat: improve file upload message handling and UI

Backend:
- Handle both string and list format for message content in uploads middleware
- Extract text content from structured message blocks
- Add logging for debugging file upload flow

Frontend:
- Separate file display from message bubble for human messages
- Show uploaded files outside the message bubble for cleaner layout
- Improve file card border styling with subtle border color
- Add debug logging for message submission with files

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/core/threads/hooks.ts

@@ -73,6 +73,12 @@ export function useSubmitThread({
   const callback = useCallback(
     async (message: PromptInputMessage) => {
       const text = message.text.trim();
+      
+      console.log('[useSubmitThread] Submitting message:', { 
+        text, 
+        hasFiles: !!message.files?.length,
+        filesCount: message.files?.length || 0 
+      });
 
       // Upload files first if any
       if (message.files && message.files.length > 0) {