fix: reserve detail cover box and async-decode list/shelf covers

- The manga detail desktop overview cover had no reserved aspect-ratio, so
  the meta column reflowed when it decoded. Give .cover aspect-ratio: 2/3 +
  object-fit: cover (mirrors MangaCard).
- Add decoding="async" to the detail hero/overview covers and the list/shelf
  covers (bookmarks, history, continue-reading, collections, tagged rows) so
  decode stays off the main thread — previously only MangaCard had it.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.115.1",
+  "version": "0.115.2",
   "private": true,
   "type": "module",
   "scripts": {