fix(theme): ensure theme.js loaded before header.js

- Add theme.js script include to header template
- Fixes theme selector not working on pages using Header component
- Ensures applyTheme() is defined before changeThemeTo() calls it

templates/header.templ

@@ -163,5 +163,6 @@ templ Header(user User, currentPath string) {
             </div>
         </div>
     </nav>
+    <script src="/static/theme.js"></script>
     <script src="/static/header.js"></script>
 }