feat(computer-use): add a headless X11/i3 environment

Give the idols-ruby and idols-kana VMs a headless computer-use
environment: Xvfb plus i3, driven by cua-driver (and computer-use-linux
as a fallback). Includes the system module (AT-SPI, fonts, Mesa, Clash
Verge proxy, proxy-region timezone), the home i3 module (Xvfb/i3/x11vnc
services, browser flags), host wiring and an eval test.

hosts/idols-kana/default.nix

@@ -18,6 +18,7 @@ in
 {
   imports = [
     ../idols-ruby/packages.nix
+    ../idols-ruby/computer-use.nix
     ../idols-ruby/oci-containers
   ];