feat(web): move Hidden tracks out of main nav, surface from Settings

Hidden tracks (M5b quarantine) is a low-frequency surface — it doesn't
warrant a permanent slot in the main nav. Moves the link under a new
'Library' section card on /settings alongside ListenBrainz config.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/components/Shell.svelte

@@ -26,7 +26,6 @@
     { href: '/library/artists', label: 'Artists' },
     { href: '/library/albums',  label: 'Albums' },
     { href: '/library/liked',   label: 'Liked' },
-    { href: '/library/hidden',  label: 'Hidden' },
     { href: '/search',          label: 'Search' },
     { href: '/discover',        label: 'Discover' },
     { href: '/requests',        label: 'Requests' },