Change the dev-mode DATA_DIR default from the relative path ./data to
$XDG_DATA_HOME/webhooker (falling back to $HOME/.local/share/webhooker).
This ensures the application's data directory does not depend on the
working directory.
Add a table to the README that clearly documents what WEBHOOKER_ENVIRONMENT
actually controls: DATA_DIR default, CORS policy, and session cookie
Secure flag.
Add tests for devDataDir() and verify the dev default is always absolute.
89af414037