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/routes/manga/[id]/+page.svelte

@@ -380,6 +380,7 @@
                     src={fileUrl(manga.cover_image_path)}
                     alt=""
                     loading="eager"
+                    decoding="async"
                 />
             {/if}
             <div class="hero-text">
@@ -401,6 +402,7 @@
                 alt="{manga.title} cover"
                 class="cover"
                 loading="eager"
+                decoding="async"
                 data-testid="manga-cover"
             />
         {/if}
@@ -791,7 +793,10 @@
 
     .cover {
         width: 100%;
-        height: auto;
+        /* Reserve the 2/3 box so the meta column doesn't reflow when the
+           cover decodes (mirrors MangaCard's cover). */
+        aspect-ratio: 2 / 3;
+        object-fit: cover;
         border-radius: var(--radius-md);
         background: var(--surface);
     }