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>