feat: ratings, shelves, sleep timer, EPUB export + fix TS errors

**Ratings (1–5 stars)**
- New `book_ratings` PB collection (session_id, user_id, slug, rating)
- `getBookRating`, `getBookAvgRating`, `setBookRating` in pocketbase.ts
- GET/POST /api/ratings/[slug] API route
- StarRating.svelte component with hover, animated stars, avg display
- Star rating shown on book detail page (desktop + mobile)

**Plan-to-Read shelf**
- `shelf` field added to `user_library` (reading/plan_to_read/completed/dropped)
- `updateBookShelf`, `getShelfMap` in pocketbase.ts
- PATCH /api/library/[slug] for shelf updates
- Shelf selector dropdown on book detail page (only when saved)
- Shelf tabs on library page to filter by category

**Sleep timer**
- `sleepUntil` state added to AudioStore
- Layout handles timer lifecycle (survives chapter navigation)
- Cycles Off → 15m → 30m → 45m → 60m → Off
- Shows live countdown in AudioPlayer when active

**EPUB export**
- Go backend: GET /api/export/{slug}?from=N&to=N
- Generates valid EPUB2 zip (mimetype uncompressed, OPF, NCX, XHTML chapters)
- Markdown → HTML via goldmark
- SvelteKit proxy at /api/export/[slug]
- Download button on book detail page (only when in library)

**Fix TS errors**
- discover/+page.svelte: currentBook possibly undefined (use {@const book})
- cardEl now $state for reactive binding

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/lib/components/AudioPlayer.svelte

@@ -681,6 +681,47 @@
 		const sec = Math.floor(s % 60);
 		return `${m}:${sec.toString().padStart(2, '0')}`;
 	}
+
+	// ── Sleep timer ────────────────────────────────────────────────────────────
+	const SLEEP_OPTIONS = [15, 30, 45, 60]; // minutes
+
+	let _tick = $state(0);
+	$effect(() => {
+		if (!audioStore.sleepUntil) return;
+		const id = setInterval(() => { _tick++; }, 1000);
+		return () => clearInterval(id);
+	});
+
+	let sleepRemainingSec = $derived.by(() => {
+		_tick; // subscribe to tick updates
+		if (!audioStore.sleepUntil) return 0;
+		return Math.max(0, Math.floor((audioStore.sleepUntil - Date.now()) / 1000));
+	});
+
+	function cycleSleepTimer() {
+		if (!audioStore.sleepUntil) {
+			// Start at first option (15 min)
+			audioStore.sleepUntil = Date.now() + SLEEP_OPTIONS[0] * 60 * 1000;
+			return;
+		}
+		const remaining = audioStore.sleepUntil - Date.now();
+		const currentMin = Math.round(remaining / 60000);
+		const idx = SLEEP_OPTIONS.findIndex(m => m >= currentMin);
+		if (idx === -1 || idx === SLEEP_OPTIONS.length - 1) {
+			// Was at max or past last — turn off
+			audioStore.sleepUntil = 0;
+		} else {
+			audioStore.sleepUntil = Date.now() + SLEEP_OPTIONS[idx + 1] * 60 * 1000;
+		}
+	}
+
+	function formatSleepRemaining(secs: number): string {
+		if (secs <= 0) return '';
+		const m = Math.floor(secs / 60);
+		const s = secs % 60;
+		if (m > 0) return `${m}m`;
+		return `${s}s`;
+	}
 </script>
 
 <svelte:window onkeydown={handleKeyDown} />
@@ -887,6 +928,24 @@
 					{m.reader_auto_next()}
 				</Button>
 				{/if}
+
+				<!-- Sleep timer -->
+				<Button
+					variant="ghost"
+					size="sm"
+					class={cn('gap-1 text-xs flex-shrink-0', audioStore.sleepUntil ? 'text-(--color-brand) bg-(--color-brand)/15 hover:bg-(--color-brand)/25' : 'text-(--color-muted)')}
+					onclick={cycleSleepTimer}
+					title={audioStore.sleepUntil ? `Sleep timer: ${formatSleepRemaining(sleepRemainingSec)} remaining` : 'Sleep timer off'}
+				>
+					<svg class="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+						<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M20.354 15.354A9 9 0 018.646 3.646 9.003 9.003 0 0012 21a9.003 9.003 0 008.354-5.646z"/>
+					</svg>
+					{#if audioStore.sleepUntil}
+						{formatSleepRemaining(sleepRemainingSec)}
+					{:else}
+						Sleep
+					{/if}
+				</Button>
 			</div>
 
 			<!-- Next chapter pre-fetch status (only when auto-next is on) -->

ui/src/lib/components/StarRating.svelte

@@ -0,0 +1,57 @@
+<script lang="ts">
+	interface Props {
+		rating: number;        // current user rating 0–5 (0 = unrated)
+		avg?: number;          // average rating
+		count?: number;        // total ratings
+		readonly?: boolean;    // display-only mode
+		size?: 'sm' | 'md';
+		onrate?: (r: number) => void;
+	}
+
+	let { rating = 0, avg = 0, count = 0, readonly = false, size = 'md', onrate }: Props = $props();
+
+	let hovered = $state(0);
+
+	const starSize = $derived(size === 'sm' ? 'w-3.5 h-3.5' : 'w-5 h-5');
+	const display = $derived(hovered || rating || 0);
+</script>
+
+<div class="flex items-center gap-1">
+	<div class="flex items-center gap-0.5">
+		{#each [1,2,3,4,5] as star}
+			<button
+				type="button"
+				disabled={readonly}
+				onmouseenter={() => { if (!readonly) hovered = star; }}
+				onmouseleave={() => { if (!readonly) hovered = 0; }}
+				onclick={() => { if (!readonly) onrate?.(star); }}
+				class="transition-transform {readonly ? 'cursor-default' : 'cursor-pointer hover:scale-110 active:scale-95'} disabled:pointer-events-none"
+				aria-label="Rate {star} star{star !== 1 ? 's' : ''}"
+			>
+				<svg
+					class="{starSize} transition-colors"
+					fill={star <= display ? 'currentColor' : 'none'}
+					stroke="currentColor"
+					stroke-width="1.5"
+					viewBox="0 0 24 24"
+				>
+					<path
+						stroke-linecap="round"
+						stroke-linejoin="round"
+						d="M11.049 2.927c.3-.921 1.603-.921 1.902 0l1.519 4.674a1 1 0 00.95.69h4.915c.969 0 1.371 1.24.588 1.81l-3.976 2.888a1 1 0 00-.363 1.118l1.518 4.674c.3.922-.755 1.688-1.538 1.118l-3.976-2.888a1 1 0 00-1.176 0l-3.976 2.888c-.783.57-1.838-.197-1.538-1.118l1.518-4.674a1 1 0 00-.363-1.118l-3.976-2.888c-.784-.57-.38-1.81.588-1.81h4.914a1 1 0 00.951-.69l1.519-4.674z"
+					/>
+				</svg>
+			</button>
+		{/each}
+	</div>
+	{#if avg && count}
+		<span class="text-xs text-(--color-muted) ml-1">{avg} ({count})</span>
+	{:else if avg}
+		<span class="text-xs text-(--color-muted) ml-1">{avg}</span>
+	{/if}
+</div>
+
+<style>
+	button[disabled] { pointer-events: none; }
+	svg { color: #f59e0b; }
+</style>