feat(post): add max-width constraint to content

Add max-w-2xl (768px) with mx-auto to center and constrain post content
width, similar to ZapWindow. Prevents content from becoming too wide on
large screens while maintaining good readability.

src/components/PostViewer.tsx

@@ -430,7 +430,7 @@ export function PostViewer() {
 
   return (
     <div className="h-full overflow-y-auto">
-      <div className="space-y-4 p-4">
+      <div className="max-w-2xl mx-auto space-y-4 p-4">
         {!showPublishedPreview ? (
           <>
             {/* Editor */}