templates: update script tags for ES modules and reader link

- Change script tags from 'defer' to 'type=module' for ES module support
- Add reader.js script to reader.templ for the reader bundle
- Update book_detail.templ 'Read Now' button to link to /readers/{id}
- Remove trailing blank line from register.templ

templates/header.templ

@@ -283,5 +283,5 @@ templ Header(user User, currentPath string) {
 			</div>
 		</div>
 	</nav>
-	<script src="/static/main.js" defer></script>
+	<script type="module" src="/static/main.js"></script>
 }