Add the agreed approach for prettifying nomarchy-menu theme into a rofi icon
grid of real themed-desktop previews: Bernardo captures previews on real
hardware and commits themes/<slug>/preview.png; a build-time derivation
downscales each to a thumbnail that the picker loads as the rofi element icon
(rofi already renders image files — same as the Papirus menu icons), with a
wallpaper/swatch/plain-name fallback while previews are added incrementally.
Notes the headless VM-render route was prototyped and works (runNixOSTest +
software-GL Hyprland + machine.screenshot QMP dump) but real-hardware capture
was chosen for fidelity; the working VM recipe is parked in memory. The
temporary flake checks.theme-shot spike is removed (too heavy for a CI gate).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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