Fix time zone issue when scheduling posts for social media

Convert scheduledDate to ISO string format in `new-post.tsx` to ensure correct time zone handling for scheduled posts.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: ae4037a0-2a6f-4530-9bac-79b543286bda
Replit-Commit-Checkpoint-Type: intermediate_checkpoint
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/397bca8c-984f-43ff-841a-10897aeb8140/ae4037a0-2a6f-4530-9bac-79b543286bda/8Nctbk0

client/src/pages/new-post.tsx

@@ -268,7 +268,7 @@ export default function NewPost() {
 
     createPostMutation.mutate({
       content: postText,
-      scheduledFor: scheduledDate || undefined,
+      scheduledFor: scheduledDate ? new Date(scheduledDate).toISOString() : undefined,
       mediaIds: selectedMedia.length > 0 ? selectedMedia : undefined,
       pageIds: selectedPages,
       postType,