feat(web): add /settings page with ListenBrainz section

Adds api.put helper, ListenBrainz API module with query/mutation
helpers, a /settings route with token + enable/disable UI, and 5
tests. Adds Settings to the Shell nav.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/lib/components/Shell.svelte

@@ -25,7 +25,8 @@
     { href: '/',              label: 'Library' },
     { href: '/library/liked', label: 'Liked' },
     { href: '/search',        label: 'Search' },
-    { href: '/playlists',     label: 'Playlists' }
+    { href: '/playlists',     label: 'Playlists' },
+    { href: '/settings',      label: 'Settings' }
   ];
 </script>