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>

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.120.0"
+version = "0.121.0"
 edition = "2021"
 default-run = "mangalord"