refactor(templates): consolidate header.js loading

- Move header.js script from individual templates to header.templ
- Removes duplicate script tags from bookshelf, collections, progress, etc.
- Fixes indentation in docs.templ

templates/header.templ

@@ -163,4 +163,5 @@ templ Header(user User, currentPath string) {
             </div>
         </div>
     </nav>
+    <script src="/static/header.js"></script>
 }