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}'

.gitea/workflows/release.yaml

@@ -239,6 +239,11 @@ jobs:
           name: ui-build-injected
           path: ui/build
 
+      - name: Allow build/ into Docker context (override .dockerignore)
+        run: |
+          grep -v '^build$' ui/.dockerignore > ui/.dockerignore.tmp
+          mv ui/.dockerignore.tmp ui/.dockerignore
+
       - uses: docker/setup-buildx-action@v3
 
       - name: Log in to Docker Hub