Add h1 prose styling for markdown # headings

h1 was missing from prose CSS, causing # headings to render
at normal text size. Added 2em/800 weight styling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Yushin-L

src/app/globals.css

@@ -33,6 +33,14 @@ body {
 }
 
 /* Prose styling for MDX content */
+.prose h1 {
+  margin-top: 2em;
+  margin-bottom: 0.75em;
+  font-size: 2em;
+  font-weight: 800;
+  line-height: 1.2;
+}
+
 .prose h2 {
   margin-top: 2em;
   margin-bottom: 0.75em;