feat: loading skeletons on the bookmarks and collections pages

Both loaders now stream the whole result (the single fetch is also the auth
gate) so the page shows a skeleton while it loads instead of the global nav
bar only: a ListRowSkeleton on bookmarks and a MangaGridSkeleton on the
collections grid, resolving to the sign-in / error / empty / list branches.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.119.0",
+  "version": "0.120.0",
   "private": true,
   "type": "module",
   "scripts": {