fix inconsistent slide nav height on first/last slides

Disabled arrow spans were missing font-size: 1.5em that active <a>
arrows had, causing the nav bar to shift vertically.

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

Author: TrevorAustin

static/css/style.css

@@ -45,7 +45,8 @@ body {
     font-family: "Droid Sans", sans-serif;
     border-bottom: 1px solid #eee;
 }
-.slide-nav a {
+.slide-nav a,
+.slide-nav .disabled {
     text-decoration: none;
     color: #333;
     font-size: 1.5em;