feat: async decode for manga cover images

Adds decoding="async" to the MangaCard cover so image decode happens off the
main thread, keeping grid scroll/paint smooth as covers stream in. The 2/3
aspect-ratio already reserves space, so no layout shift.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.113.0",
+  "version": "0.114.0",
   "private": true,
   "type": "module",
   "scripts": {