Live-reload the config on file change

[fabiosammy]

Reload deck.yml automatically when it changes, without restarting the daemon.

What to do

1. Poll the config mtime (or use a file watcher) inside the supervise loop.
2. On change: re-parse + re-render, repaint, and handle parse errors gracefully (log and keep the previous config).
3. Preserve the current layer if it still exists after reload.

Reference: deck.rb TODO header; load_config, render_layers, cmd_run.