bugfix: reader nav sticks under the app header instead of behind it (0.21.1)

$(top offset was 44px (header's 60px minus var(--space-4)), placing the bar inside the layout header. Now sticks at var(--app-header-h).)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.21.0",
+  "version": "0.21.1",
   "private": true,
   "type": "module",
   "scripts": {