Fix bottom nav bar hidden on mobile

Use 100dvh (dynamic viewport height) so the layout accounts for mobile
browser chrome (address bar, navigation bar), with 100vh as fallback.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -37,6 +37,7 @@ body {
   color: var(--text-primary);
   font-family: 'Consolas', 'Monaco', 'Courier New', monospace;
   height: 100vh;
+  height: 100dvh;
   display: flex;
   flex-direction: column;
   overflow: hidden;