Move theme toggle into header-nav for right alignment

Place the light/dark toggle button inside header-nav next to the
Prev/Next arrows for consistent right-aligned positioning.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brunoborges

docs/step.html

@@ -476,8 +476,8 @@
                 <span>🎮</span>
                 <span class="header-title">VS Code Copilot Agent Lab</span>
             </a>
-            <button class="theme-toggle" onclick="toggleTheme()">☀️ Light</button>
             <nav class="header-nav">
+                <button class="theme-toggle" onclick="toggleTheme()">☀️ Light</button>
                 <button class="nav-btn" id="prevBtn" disabled>← Previous</button>
                 <button class="nav-btn" id="nextBtn">Next →</button>
             </nav>