feat(upstream): switch to remote git repository for engine

- Update installer to use git+https://git.bemagri.xyz/bernardo/Nomarchy.git
- Remove redundant bundling/copying of the engine to /etc/nixos/nomarchy
- Expose all themes via xdg.dataFile for script accessibility
- Update theme scripts to resolve directories via local share instead of hardcoded system paths
- Update documentation to reflect the new remote-first architecture

modules/home/theme-files.nix

@@ -7,4 +7,7 @@
   };
   
   xdg.configFile."nomarchy/current/theme.name".text = config.nomarchy.theme;
+
+  # Expose all themes to the system via local share for script accessibility
+  xdg.dataFile."nomarchy/themes".source = ../../themes;
 }