refactor: move duplicates into library settings

frontend/src/routes/(root)/settings/+layout.svelte

@@ -1,9 +1,14 @@
 <script lang="ts">
 	import { page } from '$app/state';
 	import { resolve } from '$app/paths';
+	import { getLibraryState } from '$lib/state/library.svelte';
 
 	let { children } = $props();
 
+	// Set in (root)/+layout.svelte, above this group, so listing the libraries in
+	// the nav needs no load function.
+	const libraryState = getLibraryState();
+
 	// Route ids rather than paths: resolve() is called in the markup so it stays a
 	// direct call the lint rule can see, and the active check compares route ids.
 	// A pathname comparison would miss during SSR, where resolve() returns a
@@ -14,12 +19,28 @@
 		{ title: 'Libraries', routeId: '/(root)/settings/libraries' },
 		{ title: 'Devices', routeId: '/(root)/settings/devices' }
 	] as const;
+
+	/**
+	 * Whether a nested library item is the one being looked at.
+	 *
+	 * Both halves are needed: the route id alone is shared by every library, so
+	 * matching on it would light all of them up at once.
+	 */
+	function isActiveLibrary(id: number) {
+		return (
+			page.route.id?.startsWith('/(root)/settings/libraries/[libraryId]') === true &&
+			page.params.libraryId === String(id)
+		);
+	}
 </script>
 
 <div class="flex h-[calc(100vh-var(--header-height)-2rem)] flex-col">
 	<h1 class="font-serif text-xl font-medium tracking-tight">Settings</h1>
 	<div class="mt-6 flex flex-1 gap-8 overflow-hidden">
-		<nav class="flex w-48 shrink-0 flex-col gap-1">
+		<!-- Scrolls on its own now that it holds a list that grows with the number
+		     of libraries; the row above it is overflow-hidden, so without this a
+		     long list is clipped rather than reachable. -->
+		<nav class="flex w-48 shrink-0 flex-col gap-1 overflow-y-auto">
 			{#each items as item}
 				<a
 					href={resolve(item.routeId)}
@@ -30,6 +51,26 @@
 				>
 					{item.title}
 				</a>
+
+				<!-- Libraries is the one item with children: a library's own settings
+				     hang off it, so every library and every section it has stay one
+				     click from here. -->
+				{#if item.routeId === '/(root)/settings/libraries'}
+					{#each libraryState.libraries as library (library.id)}
+						<a
+							href={resolve('/(root)/settings/libraries/[libraryId]/duplicates', {
+								libraryId: String(library.id)
+							})}
+							class="ml-3 truncate rounded-md border-l px-3 py-1.5 text-sm transition-colors hover:bg-muted {isActiveLibrary(
+								library.id
+							)
+								? 'bg-muted font-medium'
+								: 'text-muted-foreground'}"
+						>
+							{library.name}
+						</a>
+					{/each}
+				{/if}
 			{/each}
 		</nav>
 		<div class="flex-1 overflow-auto">{@render children()}</div>

frontend/src/routes/(root)/settings/libraries/[libraryId]/+layout.svelte

@@ -0,0 +1,48 @@
+<script lang="ts">
+	import { page } from '$app/state';
+	import { resolve } from '$app/paths';
+	import { getLibraryState } from '$lib/state/library.svelte';
+
+	let { children } = $props();
+
+	// Set in (root)/+layout.svelte, above the settings group, so the libraries are
+	// already here — this pane needs no load function of its own.
+	const libraryState = getLibraryState();
+
+	const library = $derived(
+		libraryState.libraries.find((lib) => String(lib.id) === page.params.libraryId)
+	);
+
+	// Duplicates is the only section today. The strip exists so General and a
+	// danger zone have an obvious place to land; neither is built yet, and an
+	// empty tab is worse than no tab.
+	//
+	// Active state is matched on route id, not pathname, for the reason spelled
+	// out in settings/+layout.svelte.
+	const sections = [
+		{ title: 'Duplicates', routeId: '/(root)/settings/libraries/[libraryId]/duplicates' }
+	] as const;
+</script>
+
+<div class="flex flex-col gap-4">
+	<div>
+		<h2 class="text-lg font-semibold">{library?.name ?? 'Library'}</h2>
+		<p class="text-sm text-muted-foreground">Settings for this library</p>
+	</div>
+
+	<nav class="flex gap-1 border-b">
+		{#each sections as section (section.routeId)}
+			<a
+				href={resolve(section.routeId, { libraryId: page.params.libraryId! })}
+				class="-mb-px border-b-2 px-3 py-2 text-sm font-medium transition-colors hover:text-foreground {page
+					.route.id === section.routeId
+					? 'border-foreground'
+					: 'border-transparent text-muted-foreground'}"
+			>
+				{section.title}
+			</a>
+		{/each}
+	</nav>
+
+	{@render children()}
+</div>

frontend/src/routes/(root)/settings/libraries/[libraryId]/+page.ts

@@ -0,0 +1,10 @@
+import { resolve } from '$app/paths';
+import { redirect } from '@sveltejs/kit';
+
+export async function load({ params }) {
+	// The library pane is its sections; land on the first one.
+	redirect(
+		303,
+		resolve('/(root)/settings/libraries/[libraryId]/duplicates', { libraryId: params.libraryId })
+	);
+}

frontend/src/routes/(root)/settings/libraries/[libraryId]/duplicates/+page.svelte

@@ -70,15 +70,15 @@
 	}
 </script>
 
-<div class="mx-auto flex w-full max-w-5xl flex-col gap-6 p-4">
-	<div>
-		<h2 class="text-lg font-semibold">Possible duplicates</h2>
-		<p class="text-sm text-muted-foreground">
-			Books whose metadata matches. A second edition, a translation and a different scan of one book
-			all look like this, so nothing here has been changed or removed — this is a list to read, not
-			a problem to fix.
-		</p>
-	</div>
+<!-- No max width and no padding of its own: the settings pane sets the width and
+     is the thing that scrolls, so a wrapper that centres inside it only makes the
+     cards narrower than they need to be. -->
+<div class="flex flex-col gap-6 pb-4">
+	<p class="text-sm text-muted-foreground">
+		Books whose metadata matches. A second edition, a translation and a different scan of one book
+		all look like this, so nothing here has been changed or removed — this is a list to read, not a
+		problem to fix.
+	</p>
 
 	{#if data.groups.length === 0}
 		<Empty.Root>