Visual feedback on key press

[fabiosammy]

Briefly highlight/blink a key when pressed so there's visual confirmation on the deck itself.

What to do

1. On press, repaint that key with a highlight (e.g. brighter/inverted) for ~150 ms, then restore it.
2. Make it optional via settings.press_feedback.
3. Keep it from adding latency to the press→command path.

Reference: deck.rb TODO header; press handling in cmd_run, single-key paint.