feat: skeleton loading state on the home catalog

Replaces the plain "Loading…" text with a content-shaped MangaGridSkeleton
so the catalog grid's layout is reserved while it loads and swaps in without
a shift. Keeps the data-testid="loading" wrapper (now role="status") for
accessibility and selector stability.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.112.0",
+  "version": "0.113.0",
   "private": true,
   "type": "module",
   "scripts": {