fix: fix heading style in posts/<page-num> layouts

Heading styles not applying properly in Main.astro layout. Fix this problem by applying id selector.

src/layouts/Main.astro

@@ -17,13 +17,13 @@ const { pageTitle, pageDesc } = Astro.props;
 </main>
 
 <style>
-  main {
+  #main-content {
     @apply max-w-3xl mx-auto px-4 pb-12 w-full;
   }
-  h1 {
+  #main-content h1 {
     @apply font-semibold text-2xl sm:text-3xl;
   }
-  p {
+  #main-content p {
     @apply mt-2 mb-6 italic;
   }
 </style>