feat: add dynamic header hiding on scroll and back-to-top button

- Hide header when scrolling down, show when scrolling up
- Smooth transition animations for header hide/show
- Floating back-to-top button in bottom right corner
- Button appears after 300px scroll
- Circular button with arrow icon and smooth animations
- Mobile-responsive sizing for back-to-top button
- Both features excluded from print view

templates/index.html

@@ -248,6 +248,11 @@
         <button onclick="this.parentElement.style.display='none'" aria-label="Close error message" class="error-close">×</button>
     </div>
 
+    <!-- Back to Top Button -->
+    <button id="back-to-top" class="back-to-top no-print" aria-label="{{if eq .Lang "es"}}Volver arriba{{else}}Back to top{{end}}" style="display: none;">
+        <iconify-icon icon="mdi:arrow-up" width="24" height="24"></iconify-icon>
+    </button>
+
     <script>
         // Toggle navigation menu
         function toggleMenu() {
@@ -395,6 +400,51 @@
             }
         });
 
+        // Scroll Direction Detection - Hide/Show Header
+        let lastScrollTop = 0;
+        let scrollThreshold = 100; // Start hiding after 100px scroll
+
+        window.addEventListener('scroll', function() {
+            const actionBar = document.querySelector('.action-bar');
+            const navMenu = document.querySelector('.navigation-menu');
+            const backToTopBtn = document.getElementById('back-to-top');
+            const currentScroll = window.pageYOffset || document.documentElement.scrollTop;
+
+            // Hide/show header based on scroll direction
+            if (currentScroll > scrollThreshold) {
+                if (currentScroll > lastScrollTop) {
+                    // Scrolling down - hide header
+                    actionBar.classList.add('header-hidden');
+                    navMenu.classList.add('header-hidden');
+                } else {
+                    // Scrolling up - show header
+                    actionBar.classList.remove('header-hidden');
+                    navMenu.classList.remove('header-hidden');
+                }
+            } else {
+                // At top - always show header
+                actionBar.classList.remove('header-hidden');
+                navMenu.classList.remove('header-hidden');
+            }
+
+            // Show/hide back to top button
+            if (currentScroll > 300) {
+                backToTopBtn.style.display = 'flex';
+            } else {
+                backToTopBtn.style.display = 'none';
+            }
+
+            lastScrollTop = currentScroll <= 0 ? 0 : currentScroll;
+        }, false);
+
+        // Back to top button click handler
+        document.getElementById('back-to-top').addEventListener('click', function() {
+            window.scrollTo({
+                top: 0,
+                behavior: 'smooth'
+            });
+        });
+
         // Error handling utility
         function showError(message) {
             const errorToast = document.getElementById('error-toast');