feat(web): wire LikeButton into TrackRow, AlbumCard, ArtistRow, PlayerBar

Heart appears on every entity rendering surface. ArtistRow restructured
to a <div> with the link as a positioned overlay so the heart button
can nest without invalid <button>-in-<a> markup. Shell nav grows a
'Liked' entry pointing at /library/liked.

web/src/lib/components/Shell.svelte

@@ -22,9 +22,10 @@
   }
 
   const navItems = [
-    { href: '/',          label: 'Library' },
-    { href: '/search',    label: 'Search' },
-    { href: '/playlists', label: 'Playlists' }
+    { href: '/',              label: 'Library' },
+    { href: '/library/liked', label: 'Liked' },
+    { href: '/search',        label: 'Search' },
+    { href: '/playlists',     label: 'Playlists' }
   ];
 </script>