fix(ci): strip 'build' from .dockerignore before docker-ui build

When PREBUILT=1 the pre-built artifact is downloaded into ui/build/ but
.dockerignore excludes 'build', so Docker never sees it and /app/build
doesn't exist in the builder stage — causing the runtime COPY to fail.

Fix: rewrite ui/.dockerignore on the CI runner (grep -v '^build$') so the
pre-built directory is included in the Docker context.

Also in this commit:
- book page: gate EPUB download on isPro (UI upsell + server 403 guard)
- book page: chapter names default pattern changed to '{scene}'

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

@@ -2,7 +2,11 @@ import { error } from '@sveltejs/kit';
 import type { RequestHandler } from './$types';
 import { backendFetch } from '$lib/server/scraper';
 
-export const GET: RequestHandler = async ({ params, url }) => {
+export const GET: RequestHandler = async ({ params, url, locals }) => {
+	if (!locals.isPro) {
+		error(403, 'EPUB download requires a Pro subscription');
+	}
+
 	const { slug } = params;
 	const from = url.searchParams.get('from');
 	const to = url.searchParams.get('to');