feat: add MangaGridSkeleton loading placeholder

Composes Skeleton into a cover-grid placeholder that reuses the shared
.manga-grid / .manga-card layout, so it matches the real catalog grid at
every breakpoint and content swaps in without a 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": "0.110.0",
+  "version": "0.111.0",
   "private": true,
   "type": "module",
   "scripts": {