feat: Power of Two — digit row only level 11+, bg music, auto save/load

- Digit shortcut row (0-10) only shows from level 11 onward
- Background melody plays via IJS.audio.playMelody (C-E-G-E loop)
- Auto-save on level completion (IJS.io.write)
- Auto-load saved progress on startup (IJS.io.read)

Co-Authored-By: Claude <noreply@anthropic.com>