Some GTK themes (and newer GTK versions) leave popover surfaces
transparent. Add the canonical 'menu' class to both popovers and a
theme-aware CSS override so the difficulty/leaderboard menus always
draw an opaque background.
- io.github.bemagri.nomines.yml: builds libsigc++/glibmm/cairomm/pangomm/gtkmm
from tarballs against the GNOME 46 runtime, then the app from the v1.0.0 tag
- Icon: app-id-named PNGs (128/512) for flatpak export, svg kept for native
installs; desktop file Icon updated
- README/AGENTS.md: flatpak build instructions and version traps