update: post page layout

- Change default font style from sans to serif
- Add responsive mobile header class
- Update post images with new gallery images
- Modify post title and layout styling
- Simplify font style class application in Layout

src/pages/posts/[slug].astro

@@ -32,7 +32,7 @@ const { Content, remarkPluginFrontmatter } = await post.render()
     </h1>
 
     <div
-      class="mb-20.5 block c-primary font-time"
+      class="mb-23.4 block c-primary font-time"
       transition:name={`time-${post.data.slug || post.slug}`}
     >
       <time>