fix: subagent dropdown hidden behind terminal

Add z-index to header so dropdown menus appear above the main
content/terminal area.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/web/public/styles.css

@@ -48,6 +48,8 @@ body {
   background: var(--bg-card);
   border-bottom: 1px solid var(--border);
   flex-shrink: 0;
+  position: relative;
+  z-index: 100; /* Above main content so dropdowns work */
 }
 
 .header-brand {