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/server/pocketbase.ts

@@ -1734,6 +1734,92 @@ export async function clearDiscoveryVotes(sessionId: string, userId?: string): P
 	);
 }
 
+// ─── Ratings ──────────────────────────────────────────────────────────────────
+
+export interface BookRating {
+	session_id: string;
+	user_id?: string;
+	slug: string;
+	rating: number; // 1–5
+}
+
+export async function getBookRating(
+	sessionId: string,
+	slug: string,
+	userId?: string
+): Promise<number> {
+	const filter = userId
+		? `(session_id="${sessionId}" || user_id="${userId}") && slug="${slug}"`
+		: `session_id="${sessionId}" && slug="${slug}"`;
+	const row = await listOne<BookRating>('book_ratings', filter).catch(() => null);
+	return row?.rating ?? 0;
+}
+
+export async function getBookAvgRating(
+	slug: string
+): Promise<{ avg: number; count: number }> {
+	const rows = await listAll<BookRating>('book_ratings', `slug="${slug}"`).catch(() => []);
+	if (!rows.length) return { avg: 0, count: 0 };
+	const avg = rows.reduce((s, r) => s + r.rating, 0) / rows.length;
+	return { avg: Math.round(avg * 10) / 10, count: rows.length };
+}
+
+export async function setBookRating(
+	sessionId: string,
+	slug: string,
+	rating: number,
+	userId?: string
+): Promise<void> {
+	const filter = userId
+		? `(session_id="${sessionId}" || user_id="${userId}") && slug="${slug}"`
+		: `session_id="${sessionId}" && slug="${slug}"`;
+	const existing = await listOne<BookRating & { id: string }>('book_ratings', filter).catch(() => null);
+	const payload: Partial<BookRating> = { session_id: sessionId, slug, rating };
+	if (userId) payload.user_id = userId;
+	if (existing) {
+		await pbPatch(`/api/collections/book_ratings/records/${existing.id}`, payload);
+	} else {
+		await pbPost('/api/collections/book_ratings/records', payload);
+	}
+}
+
+// ─── Shelves ───────────────────────────────────────────────────────────────────
+
+export type ShelfName = '' | 'plan_to_read' | 'completed' | 'dropped';
+
+export async function updateBookShelf(
+	sessionId: string,
+	slug: string,
+	shelf: ShelfName,
+	userId?: string
+): Promise<void> {
+	const filter = userId
+		? `(session_id="${sessionId}" || user_id="${userId}") && slug="${slug}"`
+		: `session_id="${sessionId}" && slug="${slug}"`;
+	const existing = await listOne<{ id: string }>('user_library', filter).catch(() => null);
+	if (!existing) {
+		// Save + set shelf in one shot
+		const payload: Record<string, unknown> = { session_id: sessionId, slug, shelf, saved_at: new Date().toISOString() };
+		if (userId) payload.user_id = userId;
+		await pbPost('/api/collections/user_library/records', payload);
+	} else {
+		await pbPatch(`/api/collections/user_library/records/${existing.id}`, { shelf });
+	}
+}
+
+export async function getShelfMap(
+	sessionId: string,
+	userId?: string
+): Promise<Record<string, ShelfName>> {
+	const filter = userId
+		? `session_id="${sessionId}" || user_id="${userId}"`
+		: `session_id="${sessionId}"`;
+	const rows = await listAll<{ slug: string; shelf: string }>('user_library', filter).catch(() => []);
+	const map: Record<string, ShelfName> = {};
+	for (const r of rows) map[r.slug] = (r.shelf as ShelfName) || '';
+	return map;
+}
+
 export async function getBooksForDiscovery(
 	sessionId: string,
 	userId?: string,