fix: await marked() to prevent Promise being passed as chapter HTML

marked() returns string | Promise<string>; the previous cast 'as string'
silently passed a Promise object, which Svelte rendered as nothing.
Free users saw blank content even though SSR HTML was correct.

ui/src/routes/books/[slug]/chapters/[n]/+page.server.ts

@@ -154,7 +154,7 @@ export const load: PageServerLoad = async ({ params, url, locals }) => {
 			error(res.status === 404 ? 404 : 502, res.status === 404 ? `Chapter ${n} not found` : 'Could not fetch chapter content');
 		}
 		const markdown = await res.text();
-		html = marked(markdown) as string;
+		html = await marked(markdown);
 	} catch (e) {
 		if (e instanceof Error && 'status' in e) throw e;
 		// Don't hard-fail — show empty content with error message