RadialConfig::load() reads $XDG_CONFIG_HOME/olliepalette/radials.json
when present and non-empty, else builtinDefault(); a broken/empty file
also falls back so the user is never left without radials. Add
userConfigPath() and loadFile(path,&error). setupRadials() now calls
load(). Example at docs/radials.example.json. 4 new tests.