add config.json.example and a friendlier missing-config path

loadConfig opens ~/.config/dew/config.json directly and returns an error if it isn't there, so a fresh clone/build can't start at all and gives no hint about what to create. The struct has roughly 24 keys and none of them are documented anywhere.

Add a config.json.example (mirroring grrs's config.toml.example), and either write a default config on first run or fail with a message that points at the example.