feat: add homepage auto-redirect for logged-in users

- Add JavaScript to check for valid JWT token on homepage load
- Auto-redirect to /bookshelf if user is already logged in
- Shows login/register form if not authenticated
- Improves UX by taking logged-in users directly to bookshelf

Implementation:
- Fetch /api/auth/profile with stored token
- On success, redirect to /bookshelf
- On failure, silently stay on homepage
- Runs on DOMContentLoaded for fast execution

templates/index.templ

@@ -240,6 +240,30 @@ templ Index(loggedIn bool) {
         }
 
         <script src="/static/theme.js"></script>
+        <script>
+            // Auto-redirect to bookshelf if user is logged in
+            document.addEventListener('DOMContentLoaded', function() {
+                const token = localStorage.getItem('token');
+                if (token) {
+                    // Validate token by checking profile
+                    fetch('/api/auth/profile', {
+                        headers: {
+                            'Authorization': `Bearer ${token}`
+                        }
+                    })
+                    .then(response => {
+                        if (response.ok) {
+                            // Token is valid, redirect to bookshelf
+                            window.location.href = '/bookshelf';
+                        }
+                    })
+                    .catch(error => {
+                        // Silently fail - user stays on homepage
+                        console.log('Not logged in');
+                    });
+                }
+            });
+        </script>
     </body>
     </html>
 }