fix: serve .mjs files as application/javascript for pdfjs module worker

nginx:alpine mime.types only covers .js, not .mjs. The pdfjs-dist v4
worker is output as pdf.worker-*.mjs; without the correct MIME type the
browser refuses to execute it as a module worker and pdfjs throws
'Network Error', blocking the PDF backdrop from rendering.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -19,6 +19,9 @@ FROM nginx:alpine
 # Copy built files
 COPY --from=builder /app/dist /usr/share/nginx/html
 
+# .mjs files (pdfjs worker) must be served as application/javascript for module workers
+RUN sed -i 's|application/javascript\s*js;|application/javascript js mjs;|' /etc/nginx/mime.types
+
 # Create a simple nginx configuration
 RUN echo 'server { \
     listen 3000; \