feat: loading skeleton on the library page

The library loader streams its one-shot bundle (also the auth gate), so the
active sub-tab shows a skeleton while it loads — a grid skeleton for the
Collections tab, a row skeleton for Bookmarks/History/Page-tags — instead of
only the global nav bar.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.120.0",
+  "version": "0.121.0",
   "private": true,
   "type": "module",
   "scripts": {