fix: JXL decode fallback and Playwright remote container support

Add djxl (libjxl-tools) as primary JXL decoder with ImageMagick
fallback — fixes JXL format failures on Ubuntu where stock ImageMagick
lacks a JXL delegate. Also make Playwright Docker config respect
BASE_URL env var for testing against remote containers.

docker/Dockerfile

@@ -148,6 +148,7 @@ RUN for i in 1 2 3; do apt-get -o Acquire::Retries=3 update && break || sleep $(
     apt-get install -y --no-install-recommends \
     tini \
     imagemagick \
+    libjxl-tools \
     libraw-dev \
     libopenexr-dev \
     potrace \