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/src/lib/components/BookmarkList.svelte

@@ -23,6 +23,7 @@
                         alt=""
                         class="cover"
                         loading="lazy"
+                        decoding="async"
                     />
                 {:else}
                     <div class="cover cover-placeholder">