Scroll content inside the shell so the player shares the ambient backdrop

The player dock used to sit fixed over the scrolling page, so it needed an
opaque background. The shell is now a fixed-height frame where only the
content column scrolls, letting the dock stay transparent like the sidebar
and queue. Scroll reset/restore on navigation moves to restoreScroll(), and
the stable scrollbar gutter moves from <html> to the content scroller.

web-player/src/lib/components/layout/Sidebar.svelte

@@ -31,7 +31,7 @@
 </script>
 
 <aside
-	class="hidden shrink-0 flex-col border-r border-hairline p-5 lg:flex"
+	class="hidden min-h-0 shrink-0 flex-col border-r border-hairline p-5 lg:flex"
 	style="width:var(--mf-sidebar-w)"
 >
 	<a href="/" class="mb-4">
@@ -73,7 +73,7 @@
 			{/if}
 		</div>
 
-		<div class="flex flex-1 flex-col gap-px overflow-y-auto pb-icon-md">
+		<div class="flex min-h-0 flex-1 flex-col gap-px overflow-y-auto pb-icon-md">
 			{#each playlists as playlist (playlist.id)}
 				{@const active = page.url.pathname === `/playlists/${playlist.id}`}
 				<ListRow