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>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.115.1"
+version = "0.115.2"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.115.1"
+version = "0.115.2"
 edition = "2021"
 default-run = "mangalord"