Keep section headings in view while scrolling

Each heading sticks to the top while its section is on screen and the
next heading pushes it off. The gap above headings moves to the section
margin, since a sticky element keeps its margin in flow.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YXgsteCeqVfDQeTgyyBCcz

Author: svandragt

static/style.css

@@ -226,10 +226,14 @@ main {
   text-transform: uppercase;
   letter-spacing: .1em;
   color: var(--muted);
-  margin: 1.75rem 0 .75rem;
+  margin: 0 0 .75rem;
+  padding: .5rem 0;
+  position: sticky;
+  top: 0;
+  z-index: 1;
+  background: var(--bg);
 }
 
-.section-heading:first-child { margin-top: 0; }
 
 .project-card--pinned {
   border-color: #c7d7f9;