Run agent-browser install as coder user

The agent-browser install (which downloads Chromium) was only run as root.
Adding a second install after USER coder ensures Chromium is available
in the coder user's home directory at runtime.

https://claude.ai/code/session_01MukmmJhsNvpwRmVAqAQvd8

Author: claude

fx/Dockerfile

@@ -68,3 +68,5 @@ RUN apt update && apt dist-upgrade -y && \
 WORKDIR /home/coder
 
 USER coder
+
+RUN agent-browser install