Per-key on/off toggle state (two images)

[fabiosammy]

Let a key toggle between two states (e.g. mic muted/unmuted), each with its own image/text and command, reflecting the new state on the deck when pressed.

What to do

1. YAML: support a toggle: spec with on:/off: (image/text/command) and an optional initial state.
2. Track per-key state at runtime; on press, run the matching command and repaint just that key.
3. Optionally sync the initial state from a shell command (e.g. query current mute status).

Reference: deck.rb TODO header; render_spec and the press handler in cmd_run.