feat: add Skeleton loading-placeholder primitive

A reusable, token-driven shimmer block with configurable width/height,
radius, and variant. Decorative (aria-hidden) — loading state is announced
by the container. Freezes to a solid muted block under the global
prefers-reduced-motion override.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.109.1",
+  "version": "0.110.0",
   "private": true,
   "type": "module",
   "scripts": {