fix: viewer-pi5 use system packages for zmq/cryptography, drop cec

- python3-cec does not exist in Debian Trixie repos
- cec Python module not needed by viewer (uses cec-ctl CLI instead)
- python3-zmq and python3-cryptography installed via apt
- Strips cec/pyzmq/cryptography/netifaces from pip requirements
  to avoid Python 3.13/arm64 QEMU build failures

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

Author: Alex

docker/Dockerfile.viewer-pi5

@@ -67,7 +67,6 @@ RUN apt-get update && \
         net-tools \
         procps \
         psmisc \
-        python3-cec \
         python3-cryptography \
         python3-dev \
         python3-gi \