perf(templates): add defer attribute to main.js script tags

- Add defer attribute to all main.js script includes across 22 template files
- Improves page load performance by allowing HTML parsing to continue without blocking
- Maintains script execution order while enabling parallel resource loading

This optimization reduces page render blocking and improves perceived load times
across all admin and user-facing pages that include the main.js bundle.

Affected pages include: admin dashboard, library management, settings, user
management, analytics, bookshelf, collections, conflicts, custom sections,
devices, documentation, profile, progress tracking, queue, and authentication
pages (login/register).

templates/header.templ

@@ -261,5 +261,5 @@ templ Header(user User, currentPath string) {
 			</div>
 		</div>
 	</nav>
-	<script src="/static/main.js" defer></script>
+	<script src="/static/main.js" defer defer></script>
 }