mobile nav overflow menu + subtle kerning

Letters read tight on the site; add a small global letter-spacing with
slightly less on headings. Mobile nav was hiding the GitHub link entirely
on < sm viewports — fold overflow items into a hamburger dropdown that
closes on route change, outside click, and Escape.

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

app/globals.css

@@ -114,6 +114,11 @@ body {
   position: relative;
   background-image: linear-gradient(180deg, var(--page-shadow), transparent 28%);
   -webkit-font-smoothing: antialiased;
+  letter-spacing: 0.012em;
+}
+
+h1, h2, h3, h4, h5, h6 {
+  letter-spacing: 0.005em;
 }
 
 body::before,