[Skeyelab]

Closes #60

Summary

• Wallpaper strip is built from a single horizontal scanline at a configurable vertical position (0 = top, 100 = bottom). This replaces the previous behavior of vertically averaging every column across the full image height.
• CLI: -gradient-row (default 50), config key gradient_row.
• TUI: when wallpaper + gradient is on, a row slider (0–100) with short label; preview updates on change; value saved in config.toml.
• Validation: 0–100 for CLI and TUI config.

Notes

• Default 50 (vertical middle) is a reasonable default; configs without gradient_row still load with 50 (same as new installs).
• Behavior change for users who relied on full-height column averaging; they can tune the row to approximate the old look or pick a specific band.