feat(branding): professionalize installer and system branding

- Refactor installer with Gum for a professional interactive experience
- Create custom Nomarchy Plymouth theme with centered logo
- Establish assets/branding directory for official logos and ASCII art
- Update Fastfetch config with official ASCII art and declarative stats
- Declaratively link branding assets via Home Manager

modules/system/plymouth.nix

@@ -1,9 +1,27 @@
 { config, pkgs, ... }:
 
+let
+  nomarchy-plymouth = pkgs.stdenv.mkDerivation {
+    pname = "nomarchy-plymouth";
+    version = "1.0";
+
+    src = ../../assets/plymouth;
+
+    installPhase = ''
+      mkdir -p $out/share/plymouth/themes/nomarchy
+      cp * $out/share/plymouth/themes/nomarchy/
+      # Fix path in the plymouth file to point to the nix store if needed, 
+      # but usually Plymouth looks in its theme dir.
+      sed -i "s|/etc/plymouth/themes/nomarchy|$out/share/plymouth/themes/nomarchy|g" $out/share/plymouth/themes/nomarchy/nomarchy.plymouth
+    '';
+  };
+in
 {
   boot.initrd.systemd.enable = true;
   boot.plymouth = {
     enable = true;
+    themePackages = [ nomarchy-plymouth ];
+    theme = "nomarchy";
   };
 
   boot.kernelParams = [ "quiet" "splash" "loglevel=3" ];