ui(settings): monochrome accent, premium inputs, window resize 790x560

- Settings accent: blue shades → black/white monochrome
- Save button text uses var(--bg) for contrast
- Inputs: bottom-only border, transparent bg, no spin buttons
- Toggle: border var(--border), knob var(--bg)
- Hotkey box: transparent bg, subtle border
- Hover states on inputs and hotkey box
- Settings window: 800x580 → 790x560