style(blog): scale down in-content post headings

markdown section headings (## etc.) rendered at bootstrap defaults,
making them as large as the post title. size .blog-post h1-h4 down to
1.4/1.25/1.1/1rem so sections read as subordinate to the title.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/App.css

@@ -191,6 +191,30 @@ a.hot-pink {
   overflow-x: auto;
 }
 
+.blog-post h1,
+.blog-post h2,
+.blog-post h3,
+.blog-post h4 {
+  margin-top: 1.5rem;
+  margin-bottom: 0.5rem;
+}
+
+.blog-post h1 {
+  font-size: 1.4rem;
+}
+
+.blog-post h2 {
+  font-size: 1.25rem;
+}
+
+.blog-post h3 {
+  font-size: 1.1rem;
+}
+
+.blog-post h4 {
+  font-size: 1rem;
+}
+
 .blog-list-title {
   font-size: 1.4rem;
 }