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/routes/api/export/[slug]/+server.ts

@@ -0,0 +1,29 @@
+import { error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { backendFetch } from '$lib/server/scraper';
+
+export const GET: RequestHandler = async ({ params, url }) => {
+	const { slug } = params;
+	const from = url.searchParams.get('from');
+	const to = url.searchParams.get('to');
+
+	const qs = new URLSearchParams();
+	if (from) qs.set('from', from);
+	if (to) qs.set('to', to);
+	const query = qs.size ? `?${qs}` : '';
+
+	const res = await backendFetch(`/api/export/${encodeURIComponent(slug)}${query}`);
+
+	if (!res.ok) {
+		const text = await res.text().catch(() => '');
+		error(res.status as Parameters<typeof error>[0], text || 'Export failed');
+	}
+
+	const bytes = await res.arrayBuffer();
+	return new Response(bytes, {
+		headers: {
+			'Content-Type': 'application/epub+zip',
+			'Content-Disposition': `attachment; filename="${slug}.epub"`
+		}
+	});
+};

ui/src/routes/api/library/[slug]/+server.ts

@@ -1,6 +1,7 @@
 import { json, error } from '@sveltejs/kit';
 import type { RequestHandler } from './$types';
-import { saveBook, unsaveBook } from '$lib/server/pocketbase';
+import { saveBook, unsaveBook, updateBookShelf } from '$lib/server/pocketbase';
+import type { ShelfName } from '$lib/server/pocketbase';
 import { log } from '$lib/server/logger';
 
 /**
@@ -32,3 +33,17 @@ export const DELETE: RequestHandler = async ({ params, locals }) => {
 	}
 	return json({ ok: true });
 };
+
+/**
+ * PATCH /api/library/[slug]
+ * Update the shelf category for a saved book.
+ */
+export const PATCH: RequestHandler = async ({ params, request, locals }) => {
+	const { slug } = params;
+	const body = await request.json().catch(() => null);
+	const shelf = body?.shelf ?? '';
+	const VALID = ['', 'plan_to_read', 'completed', 'dropped'];
+	if (!VALID.includes(shelf)) error(400, 'invalid shelf');
+	await updateBookShelf(locals.sessionId, slug, shelf as ShelfName, locals.user?.id);
+	return json({ ok: true });
+};

ui/src/routes/api/ratings/[slug]/+server.ts

@@ -0,0 +1,24 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { getBookRating, getBookAvgRating, setBookRating } from '$lib/server/pocketbase';
+
+export const GET: RequestHandler = async ({ params, locals }) => {
+	const { slug } = params;
+	const [userRating, avg] = await Promise.all([
+		getBookRating(locals.sessionId, slug, locals.user?.id),
+		getBookAvgRating(slug)
+	]);
+	return json({ userRating, avg: avg.avg, count: avg.count });
+};
+
+export const POST: RequestHandler = async ({ params, request, locals }) => {
+	const { slug } = params;
+	const body = await request.json().catch(() => null);
+	const rating = body?.rating;
+	if (typeof rating !== 'number' || rating < 1 || rating > 5) {
+		error(400, 'rating must be 1–5');
+	}
+	await setBookRating(locals.sessionId, slug, rating, locals.user?.id);
+	const avg = await getBookAvgRating(slug);
+	return json({ ok: true, avg: avg.avg, count: avg.count });
+};