fix: surface reader load failures instead of spinning forever

The PDF reader pointed an iframe at the vendored pdf.js viewer and had no
way to report a failure: the viewer renders its own errors inside the
frame, so a missing or unreadable file showed an empty grey panel. Probe
the file with a request first and render an error card with a retry when
that fails, matching the chrome the EPUB reader uses.

Add the missing +page.ts for the PDF route so the tab gets the book title
rather than a bare "Reader".

Drop chapter-sidebar.svelte and its import. It rendered a list of chapters
whose buttons had no click handler, and read/+layout@.svelte imported it
without ever rendering it. The reader's own sidebar replaces it.

frontend/src/lib/components/reader/chapter-sidebar.svelte

@@ -1,27 +0,0 @@
-<script>
-	import * as Sidebar from '$lib/components/ui/sidebar/index';
-
-	let { chapters } = $props();
-</script>
-
-<Sidebar.Root>
-	<Sidebar.Header />
-	<Sidebar.Content>
-		<Sidebar.GroupLabel>Chapters</Sidebar.GroupLabel>
-		<Sidebar.GroupContent>
-			<Sidebar.Menu>
-				{#each chapters as chapter}
-					<Sidebar.MenuItem>
-						<Sidebar.MenuButton>
-							{#snippet child({ props })}
-								<span>{chapter.label}</span>
-							{/snippet}
-						</Sidebar.MenuButton>
-					</Sidebar.MenuItem>
-				{/each}
-			</Sidebar.Menu>
-		</Sidebar.GroupContent>
-		<Sidebar.Group />
-	</Sidebar.Content>
-	<Sidebar.Footer />
-</Sidebar.Root>

frontend/src/routes/(root)/(library)/book/[bookId]/read/+layout@.svelte

@@ -1,7 +1,4 @@
 <script lang="ts">
-	import * as Sidebar from '$lib/components/ui/sidebar/index.js';
-	import ChapterSidebar from '$lib/components/reader/chapter-sidebar.svelte';
-
 	let { children } = $props();
 </script>
 

frontend/src/routes/(root)/(library)/book/[bookId]/read/pdf/[fileId]/+page.svelte

@@ -1,42 +1,99 @@
 <script lang="ts">
 	import { page } from '$app/state';
+	import { Button, buttonVariants } from '$lib/components/ui/button/index.js';
+	import { Spinner } from '$lib/components/ui/spinner/index';
+	import ThemeToggle from '$lib/components/layout/theme-toggle.svelte';
+	import { RotateCcw, TriangleAlert } from '@lucide/svelte';
 
-	let fileId = page.params.fileId;
+	let { data } = $props();
-	let bookId = page.params.bookId;
 
-	// Make sure this endpoint returns the complete PDF file
+	const fileId = page.params.fileId;
-	let url = `${page.url.origin}/api/books/download/${bookId}/${fileId}`;
+	const bookId = page.params.bookId;
+
+	const fileUrl = `${page.url.origin}/api/books/download/${bookId}/${fileId}`;
 
 	let iframeEl = $state<HTMLIFrameElement>();
+	let loading = $state(true);
+	let loadError = $state<string | null>(null);
+	let attempt = $state(0);
+
+	async function load() {
+		loading = true;
+		loadError = null;
+
+		try {
+			// Ask for the file before handing it to pdf.js. The viewer runs in an
+			// iframe and reports its own failures inside that frame, so without
+			// this probe a missing file just showed an empty grey panel.
+			const response = await fetch(fileUrl, { method: 'HEAD' });
+
+			if (!response.ok) {
+				throw new Error(`The server returned ${response.status} for this file.`);
+			}
+
+			if (iframeEl) {
+				iframeEl.src = `/pdfjs/web/viewer.html?file=${encodeURIComponent(fileUrl)}`;
+			}
+		} catch (error) {
+			console.error('Error loading PDF', error);
+			loadError = error instanceof Error ? error.message : 'The file could not be opened.';
+			loading = false;
+		}
+	}
 
 	$effect(() => {
-		if (iframeEl && url) {
+		attempt;
-			// Use relative path to viewer.html with the PDF file URL as a parameter
+		if (iframeEl) load();
-			iframeEl.src = `/pdfjs/web/viewer.html?file=${encodeURIComponent(url)}`;
-		}
 	});
 </script>
 
-<div class="pdf-container">
+<svelte:head>
-	<iframe bind:this={iframeEl} title="PDF Viewer" class="pdf-iframe"></iframe>
+	<title>{data?.title ? `${data.title} — Chitai` : 'Reader — Chitai'}</title>
+</svelte:head>
+
+<main class="flex h-screen w-full flex-col overflow-hidden bg-background">
+	<!-- Same chrome as the EPUB reader, so leaving works the same way in both -->
+	<header class="flex h-12 shrink-0 items-center gap-2 border-b px-3">
+		<a
+			href="/book/{bookId}"
+			class="min-w-0 flex-1 truncate font-serif text-sm hover:underline"
+			title={data?.title}
+		>
+			{data?.title || 'Reader'}
+		</a>
+		<ThemeToggle />
+	</header>
+
+	<div class="relative flex-1">
+		{#if loadError}
+			<div class="absolute inset-0 flex items-center justify-center p-6">
+				<div class="flex max-w-sm flex-col items-center gap-3 text-center">
+					<TriangleAlert class="size-8 text-destructive" />
+					<h2 class="font-serif text-lg">This file wouldn't open</h2>
+					<p class="text-sm text-muted-foreground">{loadError}</p>
+					<div class="mt-2 flex gap-2">
+						<Button onclick={() => attempt++}>
+							<RotateCcw class="size-4" />
+							Try again
+						</Button>
+						<a href="/book/{bookId}" class={buttonVariants({ variant: 'outline' })}>
+							Back to book
+						</a>
 					</div>
+				</div>
+			</div>
+		{:else if loading}
+			<div class="absolute inset-0 flex items-center justify-center gap-3">
+				<Spinner />
+				<span class="text-sm text-muted-foreground">Opening…</span>
+			</div>
+		{/if}
 
-<style>
+		<iframe
-	/* Make the container take up the full viewport height */
+			bind:this={iframeEl}
-	.pdf-container {
+			title="PDF viewer"
-		position: fixed;
+			class="h-full w-full border-0 {loading || loadError ? 'invisible' : ''}"
-		top: 0;
+			onload={() => (loading = false)}
-		left: 0;
+		></iframe>
-		right: 0;
+	</div>
-		bottom: 0;
+</main>
-		width: 100%;
-		height: 100vh;
-	}
-
-	/* Make the iframe fill its container */
-	.pdf-iframe {
-		width: 100%;
-		height: 100%;
-		border: none;
-	}
-</style>

frontend/src/routes/(root)/(library)/book/[bookId]/read/pdf/[fileId]/+page.ts

@@ -0,0 +1,13 @@
+import { error } from '@sveltejs/kit';
+
+export async function load({ fetch, params }) {
+	const response = await fetch(`/api/books/${params.bookId}`);
+
+	if (!response.ok) {
+		error(response.status, 'This book could not be loaded.');
+	}
+
+	const book = await response.json();
+
+	return { title: book.title };
+}