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/state/navigation.svelte.ts

@@ -1,4 +1,4 @@
-import { afterNavigate } from '$app/navigation';
+import { afterNavigate, beforeNavigate } from '$app/navigation';
 import type { LucideIcon } from '@lucide/svelte';
 import Home from '@lucide/svelte/icons/house';
 import Compass from '@lucide/svelte/icons/compass';
@@ -55,6 +55,26 @@ export function trackNavigation() {
 	});
 }
 
+const scrollPositions = new Map<number, number>();
+
+export function restoreScroll(getScroller: () => HTMLElement | undefined) {
+	let current: number | null = null;
+
+	beforeNavigate(() => {
+		const scroller = getScroller();
+		if (current !== null && scroller) scrollPositions.set(current, scroller.scrollTop);
+	});
+
+	afterNavigate((nav) => {
+		current = historyIndex();
+		const scroller = getScroller();
+		if (!scroller || nav.type === 'enter' || nav.to?.url.hash) return;
+		const saved =
+			nav.type === 'popstate' && current !== null ? scrollPositions.get(current) : undefined;
+		scroller.scrollTop = saved ?? 0;
+	});
+}
+
 export function previousPage(): PageRef | null {
 	return visited.get(index - 1) ?? null;
 }