Fix glob CVE-2025-64756 security vulnerability in Docker images

Upgrade npm to latest version in both backend and frontend Dockerfiles
to fix the command injection vulnerability in glob's CLI (CVE-2025-64756).
The vulnerability exists in npm's bundled glob package (< 10.5.0 or < 11.1.0).

frontend/Dockerfile

@@ -12,6 +12,9 @@ LABEL org.opencontainers.image.source="https://github.com/the-luap/picpeak"
 LABEL org.opencontainers.image.description="PicPeak Frontend Application"
 LABEL org.opencontainers.image.licenses="MIT"
 
+# Upgrade npm to fix glob CVE-2025-64756 vulnerability
+RUN npm install -g npm@latest
+
 # Set working directory
 WORKDIR /app